Prose relay ue activation

ABSTRACT

The present disclosure relates to a method for activating a relay functionality of a ProSe capable and relay-capable user equipment within a mobile communication network. The radio base station, to which the relay UE is connected, determines whether further relays are necessary in the radio cell controlled by the radio base station. In case further relays are necessary in the radio cell, the radio base station selects a persistence check value and transmits a broadcast message in the radio cell. The broadcast message at least indicates that further relays are necessary and comprises the selected persistence check value. Upon receiving the broadcast message, the relay UE activates its relay functionality in case it determines that relay requirements for activating its relay functionality in the radio cell are fulfilled and in case a persistence check performed by the relay UE based on the received persistence check value is successful.

BACKGROUND Technical Field

The present disclosure relates to methods for activating a relayfunctionality of a relay user equipment. The present disclosure is alsoproviding the relay user equipment and corresponding radio base stationfor participating in the methods described herein.

Description of the Related Art Long Term Evolution (LTE)

Third-generation mobile systems (3G) based on WCDMA radio-accesstechnology are being deployed on a broad scale all around the world. Afirst step in enhancing or evolving this technology entails introducingHigh-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, alsoreferred to as High Speed Uplink Packet Access (HSUPA), giving a radioaccess technology that is highly competitive.

In order to be prepared for further increasing user demands and to becompetitive against new radio access technologies, 3GPP introduced a newmobile communication system which is called Long Term Evolution (LTE).LTE is designed to meet the carrier needs for high speed data and mediatransport as well as high capacity voice support for the next decade.The ability to provide high bit rates is a key measure for LTE.

The work item (WI) specification on Long-Term Evolution (LTE) calledEvolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial RadioAccess Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTEsystem represents efficient packet-based radio access and radio accessnetworks that provide full IP-based functionalities with low latency andlow cost. In LTE, scalable multiple transmission bandwidths arespecified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order toachieve flexible system deployment using a given spectrum. In thedownlink, Orthogonal Frequency Division Multiplexing (OFDM)-based radioaccess was adopted because of its inherent immunity to multipathinterference (MPI) due to a low symbol rate, the use of a cyclic prefix(CP) and its affinity to different transmission bandwidth arrangements.Single-carrier frequency division multiple access (SC-FDMA)-based radioaccess was adopted in the uplink, since provisioning of wide areacoverage was prioritized over improvement in the peak data rateconsidering the restricted transmit power of the user equipment (UE).Many key packet radio access techniques are employed includingmultiple-input multiple-output (MIMO) channel transmission techniquesand a highly efficient control signaling structure is achieved in LTERel. 8/9.

LTE Architecture

The overall LTE architecture is shown in FIG. 1. The E-UTRAN consists ofan eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) andcontrol plane (RRC) protocol terminations towards the user equipment(UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control(MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP)layers that include the functionality of user-plane header compressionand encryption. It also offers Radio Resource Control (RRC)functionality corresponding to the control plane. It performs manyfunctions including radio resource management, admission control,scheduling, enforcement of negotiated uplink Quality of Service (QoS),cell information broadcast, ciphering/deciphering of user and controlplane data, and compression/decompression of downlink/uplink user planepacket headers. The eNodeBs are interconnected with each other by meansof the X2 interface.

The eNodeBs are also connected by means of the S1 interface to the EPC(Evolved Packet Core), more specifically to the MME (Mobility ManagementEntity) by means of the S1-MME and to the Serving Gateway (SGW) by meansof the S1-U. The S1 interface supports a many-to-many relation betweenMMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user datapackets, while also acting as the mobility anchor for the user planeduring inter-eNodeB handovers and as the anchor for mobility between LTEand other 3GPP technologies (terminating S4 interface and relaying thetraffic between 2G/3G systems and PDN GW). For idle-state userequipments, the SGW terminates the downlink data path and triggerspaging when downlink data arrives for the user equipment. It manages andstores user equipment contexts, e.g., parameters of the IP bearerservice, or network internal routing information. It also performsreplication of the user traffic in case of lawful interception.

The MME is the key control-node for the LTE access-network. It isresponsible for idle-mode user equipment tracking and paging procedureincluding retransmissions. It is involved in the beareractivation/deactivation process and is also responsible for choosing theSGW for a user equipment at the initial attach and at the time ofintra-LTE handover involving Core Network (CN) node relocation. It isresponsible for authenticating the user (by interacting with the HSS).The Non-Access Stratum (NAS) signaling terminates at the MME, and it isalso responsible for the generation and allocation of temporaryidentities to user equipments. It checks the authorization of the userequipment to camp on the service provider's Public Land Mobile Network(PLMN) and enforces user equipment roaming restrictions. The MME is thetermination point in the network for ciphering/integrity protection forNAS signaling and handles the security key management. Lawfulinterception of signaling is also supported by the MME. The MME alsoprovides the control plane function for mobility between LTE and 2G/3Gaccess networks with the S3 interface terminating at the MME from theSGSN. The MME also terminates the Sha interface towards the home HSS forroaming user equipments.

Component Carrier Structure in LTE

The downlink component carrier of a 3GPP LTE system is subdivided in thetime-frequency domain in so-called subframes. In 3GPP LTE each subframeis divided into two downlink slots as shown in FIG. 2, wherein the firstdownlink slot comprises the control channel region (PDCCH region) withinthe first OFDM symbols. Each subframe consists of a give number of OFDMsymbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release8)), wherein each OFDM symbol spans over the entire bandwidth of thecomponent carrier. The OFDM symbols thus each consist of a number ofmodulation symbols transmitted on respective subcarriers. In LTE, thetransmitted signal in each slot is described by a resource grid ofN^(DL) _(RB)*N^(RB) _(SC) subcarriers and N^(DL) _(symb) OFDM symbols.N^(DL) _(RB) is the number of resource blocks within the bandwidth. Thequantity NDLRB depends on the downlink transmission bandwidth configuredin the cell and shall fulfill

N _(RB) ^(min,DL) ≤N _(RB) ^(DL) ≤N _(RB) ^(max,DL),

where N^(min,DL) _(RB)=6 and N^(max,DL) _(RB)=110 are respectively thesmallest and the largest downlink bandwidths, supported by the currentversion of the specification. N^(RB) _(SC) is the number of subcarrierswithin one resource block. For normal cyclic prefix subframe structure,N^(RB) _(SC)=12 and N^(DL) _(symb)=7.

Assuming a multi-carrier communication system, e.g., employing OFDM, asfor example used in 3GPP Long Term Evolution (LTE), the smallest unit ofresources that can be assigned by the scheduler is one “resource block”.A physical resource block (PRB) is defined as consecutive OFDM symbolsin the time domain (e.g., 7 OFDM symbols) and consecutive subcarriers inthe frequency domain as exemplified in FIG. 2 (e.g., 12 subcarriers fora component carrier). In 3GPP LTE (Release 8), a physical resource blockthus consists of resource elements, corresponding to one slot in thetime domain and 180 kHz in the frequency domain (for further details onthe downlink resource grid, see for example 3GPP TS 36.211, “EvolvedUniversal Terrestrial Radio Access (E-UTRA); Physical Channels andModulation (Release 8)”, current version 12.6.9, section 6.2, availableat http://www.3gpp.org and incorporated herein by reference).

One subframe consists of two slots, so that there are 14 OFDM symbols ina subframe when a so-called “normal” CP (cyclic prefix) is used, and 12OFDM symbols in a subframe when a so-called “extended” CP is used. Forsake of terminology, in the following the time-frequency resourcesequivalent to the same consecutive subcarriers spanning a full subframeis called a “resource block pair”, or equivalent “RB pair” or “PRBpair”.

The term “component carrier” refers to a combination of several resourceblocks in the frequency domain. In future releases of LTE, the term“component carrier” is no longer used; instead, the terminology ischanged to “cell”, which refers to a combination of downlink andoptionally uplink resources. The linking between the carrier frequencyof the downlink resources and the carrier frequency of the uplinkresources is indicated in the system information transmitted on thedownlink resources.

Similar assumptions for the component carrier structure will apply tolater releases too.

Carrier Aggregation in LTE-A for support of wider bandwidth

The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequencyspectrum for IMT-Advanced was decided, the actual available frequencybandwidth is different according to each region or country. Followingthe decision on the available frequency spectrum outline, however,standardization of a radio interface started in the 3rd GenerationPartnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the StudyItem description on “Further Advancements for E-UTRA (LTE-Advanced)” wasapproved. The study item covers technology components to be consideredfor the evolution of E-UTRA, e.g., to fulfill the requirements onIMT-Advanced.

The bandwidth that the LTE-Advanced system is able to support is 100MHz, while an LTE system can only support 20 MHz. Nowadays, the lack ofradio spectrum has become a bottleneck of the development of wirelessnetworks, and as a result it is difficult to find a spectrum band whichis wide enough for the LTE-Advanced system. Consequently, it is urgentto find a way to gain a wider radio spectrum band, wherein a possibleanswer is the carrier aggregation functionality.

In carrier aggregation, two or more component carriers are aggregated inorder to support wider transmission bandwidths up to 100 MHz. Severalcells in the LTE system are aggregated into one wider channel in theLTE-Advanced system which is wide enough for 100 MHz even though thesecells in LTE may be in different frequency bands.

All component carriers can be configured to be LTE Rel. 8/9 compatible,at least when the bandwidth of a component carrier does not exceed thesupported bandwidth of an LTE Rel. 8/9 cell. Not all component carriersaggregated by a user equipment may necessarily be Rel. 8/9 compatible.Existing mechanisms (e.g., barring) may be used to avoid Rel-8/9 userequipments to camp on a component carrier.

A user equipment may simultaneously receive or transmit on one ormultiple component carriers (corresponding to multiple serving cells)depending on its capabilities. An LTE-A Rel. 10 user equipment withreception and/or transmission capabilities for carrier aggregation cansimultaneously receive and/or transmit on multiple serving cells,whereas an LTE Rel. 8/9 user equipment can receive and transmit on asingle serving cell only, provided that the structure of the componentcarrier follows the Rel. 8/9 specifications.

Carrier aggregation is supported for both contiguous and non-contiguouscomponent carriers with each component carrier limited to a maximum of110 Resource Blocks in the frequency domain (using the 3GPP LTE (Release8/9) numerology).

It is possible to configure a 3GPP LTE-A (Release 10)-compatible userequipment to aggregate a different number of component carriersoriginating from the same eNodeB (base station) and of possiblydifferent bandwidths in the uplink and the downlink. The number ofdownlink component carriers that can be configured depends on thedownlink aggregation capability of the UE. Conversely, the number ofuplink component carriers that can be configured depends on the uplinkaggregation capability of the UE. It may currently not be possible toconfigure a mobile terminal with more uplink component carriers thandownlink component carriers. In a typical TDD deployment the number ofcomponent carriers and the bandwidth of each component carrier in uplinkand downlink is the same. Component carriers originating from the sameeNodeB need not provide the same coverage.

The spacing between center frequencies of contiguously aggregatedcomponent carriers shall be a multiple of 300 kHz. This is in order tobe compatible with the 100 kHz frequency raster of 3GPP LTE (Release8/9) and at the same time to preserve orthogonality of the subcarrierswith 15 kHz spacing. Depending on the aggregation scenario, the n*300kHz spacing can be facilitated by insertion of a low number of unusedsubcarriers between contiguous component carriers.

The nature of the aggregation of multiple carriers is only exposed up tothe MAC layer. For both uplink and downlink there is one HARQ entityrequired in MAC for each aggregated component carrier. There is (in theabsence of SU-MIMO for uplink) at most one transport block per componentcarrier. A transport block and its potential HARQ retransmissions needto be mapped on the same component carrier.

When carrier aggregation is configured, the mobile terminal only has oneRRC connection with the network. At RRC connectionestablishment/re-establishment, one cell provides the security input(one ECGI, one PCI and one ARFCN) and the non-access stratum mobilityinformation (e.g., TAI) similarly as in LTE Rel. 8/9. After RRCconnection establishment/re-establishment, the component carriercorresponding to that cell is referred to as the downlink Primary Cell(PCell). There is always one and only one downlink PCell (DL PCell) andone uplink PCell (UL PCell) configured per user equipment in connectedstate. Within the configured set of component carriers, other cells arereferred to as Secondary Cells (SCells); with carriers of the SCellbeing the Downlink Secondary Component Carrier (DL SCC) and UplinkSecondary Component Carrier (UL SCC). Maximum five serving cells,including the PCell, can be configured for one UE.

The characteristics of the downlink and uplink PCell are:

1. For each SCell the usage of uplink resources by the UE in addition tothe downlink ones is configurable (the number of DL SCCs configured istherefore always larger or equal to the number of UL SCCs, and no SCellcan be configured for usage of uplink resources only)

2. The downlink PCell cannot be de-activated, unlike SCells

3. Re-establishment is triggered when the downlink PCell experiencesRayleigh fading (RLF), not when downlink SCells experience RLF

4. Non-access stratum information is taken from the downlink PCell

5. PCell can only be changed with handover procedure (i.e., withsecurity key change and RACH procedure)

6. PCell is used for transmission of PUCCH

7. The uplink PCell is used for transmission of Layer 1 uplink controlinformation

8. From a UE viewpoint, each uplink resource only belongs to one servingcell

The configuration and reconfiguration, as well as addition and removal,of component carriers can be performed by RRC. Activation anddeactivation is done via MAC control elements. At intra-LTE handover,RRC can also add, remove, or reconfigure SCells for usage in the targetcell. When adding a new SCell, dedicated RRC signaling is used forsending the system information of the SCell, the information beingnecessary for transmission/reception (similarly as in Rel-8/9 forhandover). Each SCell is configured with a serving cell index, when theSCell is added to one UE; PCell has always the serving cell index 0.

When a user equipment is configured with carrier aggregation there is atleast one pair of uplink and downlink component carriers that is alwaysactive. The downlink component carrier of that pair might be alsoreferred to as ‘DL anchor carrier’. Same applies also for the uplink.

When carrier aggregation is configured, a user equipment may bescheduled on multiple component carriers simultaneously, but at most onerandom access procedure shall be ongoing at any time. Cross-carrierscheduling allows the PDCCH of a component carrier to schedule resourceson another component carrier. For this purpose a component carrieridentification field is introduced in the respective DCI (DownlinkControl Information) formats, called CIF.

A linking, established by RRC signaling, between uplink and downlinkcomponent carriers allows identifying the uplink component carrier forwhich the grant applies when there is no cross-carrier scheduling. Thelinkage of downlink component carriers to uplink component carrier doesnot necessarily need to be one to one. In other words, more than onedownlink component carrier can link to the same uplink componentcarrier. At the same time, a downlink component carrier can only link toone uplink component carrier.

LTE Device to Device (D2D) Proximity Services (ProSe)

Proximity-based applications and services represent an emergingsocial-technological trend. The identified areas include servicesrelated to commercial services and Public Safety that would be ofinterest to operators and users. The introduction of a ProximityServices (ProSe) capability in LTE would allow the 3GPP industry toserve this developing market and will, at the same time, serve theurgent needs of several Public Safety communities that are jointlycommitted to LTE.

Device-to-Device (D2D) communication is a technology componentintroduced by LTE-Rel.12, which allows D2D as an underlay to thecellular network to increase the spectral efficiency. For example, ifthe cellular network is LTE, all data-carrying physical channels useSC-FDMA for D2D signaling. In D2D communications, user equipmentstransmit data signals to each other over a direct link using thecellular resources instead of through the radio base station. Throughoutthe present disclosure the terms “D2D”, “ProSe” and “sidelink” areinterchangeable.

D2D communication in LTE

The D2D communication in LTE is focusing on two areas: Discovery andCommunication.

ProSe (Proximity-based Services) Direct Discovery is defined as theprocedure used by the ProSe-enabled UE to discover other ProSe-enabledUE(s) in its proximity using E-UTRA direct radio signals via the PC5interface and will be described in more detail later.

In D2D communication, UEs transmit data signals to each other over adirect link using the cellular resources instead of through the basestation (BS). D2D users communicate directly while remaining controlledunder the BS, i.e., at least when being in coverage of an eNB.Therefore, D2D can improve system performances by reusing cellularresources.

It is assumed that D2D operates in the uplink LTE spectrum (in the caseof FDD) or uplink sub-frames of the cell giving coverage (in case ofTDD, except when out of coverage). Furthermore, D2Dtransmission/reception does not use full duplex on a given carrier. Fromindividual UE perspective, on a given carrier D2D signal reception andLTE uplink transmission do not use full duplex, i.e., no simultaneousD2D signal reception and LTE UL transmission is possible.

In D2D communication, when one particular UE1 has a role of transmission(transmitting user equipment or transmitting terminal), UE1 sends data,and another UE2 (receiving user equipment) receives it. UE1 and UE2 canchange their transmission and reception role. The transmission from UE1can be received by one or more UEs like UE2.

With respect to the user plane protocols, part of the agreement from D2Dcommunication perspective is given in the following (see also 3GPP TR36.843 current version 12.0.1 section 9.2.2, incorporated herein byreference):

-   -   PDCP:        -   1:M D2D broadcast communication data (i.e., IP packets)            should be handled as the normal user-plane data.        -   Header-compression/decompression in PDCP is applicable for            1:M D2D broadcast communication.            -   U-Mode is used for header compression in PDCP for D2D                broadcast operation for public safety;    -   RLC:        -   RLC UM is used for 1:M D2D broadcast communication.        -   Segmentation and Re-assembly is supported on L2 by RLC UM.        -   A receiving UE needs to maintain at least one RLC UM entity            per transmitting peer UE.        -   An RLC UM receiver entity does not need to be configured            prior to reception of the first RLC UM data unit.        -   So far no need has been identified for RLC AM or RLC TM for            D2D communication for user plane data transmission.    -   MAC:        -   No HARQ feedback is assumed for 1:M D2D broadcast            communication        -   The receiving UE needs to know a source ID in order to            identify the receiver RLC UM entity.        -   The MAC header comprises a L2 target ID which allows            filtering out packets at MAC layer.        -   The L2 target ID may be a broadcast, group cast or unicast            address.            -   L2 Groupcast/Unicast: A L2 target ID carried in the MAC                header would allow discarding a received RLC UM PDU even                before delivering it to the RLC receiver entity.            -   L2 Broadcast: A receiving UE would process all received                RLC PDUs from all transmitters and aim to re-assemble                and deliver IP packets to upper layers.        -   MAC sub header contains LCIDs (to differentiate multiple            logical channels).        -   At least Multiplexing/de-multiplexing, priority handling and            padding are useful for D2D.            ProSe direct communication layer-2 link

In brief, ProSe direct one-to-one communication is realized byestablishing a secure layer-2 link over PC5 between two UEs. Each UE hasa Layer-2 ID for unicast communication that is included in the SourceLayer-2 ID field of every frame that it sends on the layer-2 link and inthe Destination Layer-2 ID of every frame that it receives on thelayer-2 link. The UE needs to ensure that the Layer-2 ID for unicastcommunication is at least locally unique. So the UE should be preparedto handle Layer-2 ID conflicts with adjacent UEs using unspecifiedmechanisms (e.g., self-assign a new Layer-2 ID for unicast communicationwhen a conflict is detected). The layer-2 link for ProSe directcommunication one-to-one is identified by the combination of the Layer-2IDs of the two UEs. This means that the UE can engage in multiplelayer-2 links for ProSe direct communication one-to-one using the sameLayer-2 ID.

ProSe direct communication one-to-one is composed of the followingprocedures as explained in detail in 3GPP TR 23.713 current versionv1.4.0 section 7.1.2 incorporated herein by reference:

-   -   Establishment of a secure layer-2 link over PC5.    -   IP address/prefix assignment.    -   Layer-2 link maintenance over PC5.    -   Layer-2 link release over PC5.

FIG. 3 discloses how to establish a secure layer-2 link over the PC5interface.

1. UE-1 sends a Direct Communication Request message to UE-2 in order totrigger mutual authentication. The link initiator (UE-1) needs to knowthe Layer-2 ID of the peer (UE-2) in order to perform step 1. As anexample, the link initiator may learn the Layer-2 ID of the peer byexecuting a discovery procedure first or by having participated in ProSeone-to-many communication including the peer.

2. UE-2 initiates the procedure for mutual authentication. Thesuccessful completion of the authentication procedure completes theestablishment of the secure layer-2 link over PC5.

At least the following standard IETF mechanisms can be used for IPaddress/prefix assignment:

-   -   DHCP-based IP address configuration for assignment of an IPv4        address.    -   IPv6Stateless Address auto configuration specified in RFC 4862        for assignment of an IPv6 prefix.

One of the two UEs acts as a DHCP server or an IPv6 default router. Inthe ProSe UE-NW Relay case (also see later chapter on ProSe relay), therelay acts as DHCP server or IPv6 default router for all Remote UEs thatconnect to it over a secure layer-2 link over PC5.

UEs engaging in isolated (non-relay) one-to-one communication may alsouse link-local addresses.

The PC5 Signaling Protocol shall support keep-alive functionality thatis used to detect when the UEs are not in ProSe Communication range, sothat they can proceed with implicit layer-2 link release.

The Layer-2 link release over the PC5 can be performed by using aDisconnect Request message transmitted to the other UE, which alsodeletes all associated context data. Upon reception of the DisconnectRequest message, the other UE responds with a Disconnect Responsemessage and deletes all context data associated with the layer-2 link.

ProSe Direct Communication Related identities

3GPP TS 36.300, current version 13.3.0 defines in subclause 8.3 thefollowing identities to use for ProSe Direct Communication:

-   -   SL-RNTI: Unique identification used for ProSe Direct        Communication Scheduling;    -   Source Layer-2 ID: Identifies the sender of the data in sidelink        ProSe Direct Communication. The Source Layer-2 ID is 24 bits        long and is used together with ProSe Layer-2 Destination ID and        LCD for identification of the RLC UM entity and PDCP entity in        the receiver;    -   Destination Layer-2 ID: Identifies the target of the data in        sidelink ProSe Direct Communication. The Destination Layer-2 ID        is 24 bits long and is split in the MAC layer into two bit        strings:        -   One bit string is the LSB part (8 bits) of Destination            Layer-2 ID and forwarded to the physical layer as Sidelink            Control Layer-1 ID. This identifies the target of the            intended data in Sidelink Control and is used for filtering            packets at the physical layer.        -   Second bit string is the MSB part (16 bits) of the            Destination Layer-2 ID and is carried within the MAC header.            This is used for filtering packets at the MAC layer.

No Access Stratum signaling is required for group formation and toconfigure Source Layer-2 ID, Destination Layer-2 ID and Sidelink ControlL1 ID in the UE. These identities are either provided by a higher layeror derived from identities provided by a higher layer. In case ofgroupcast and broadcast, the ProSe UE ID provided by the higher layer isused directly as the Source Layer-2 ID, and the ProSe Layer-2 Group IDprovided by the higher layer is used directly as the Destination Layer-2ID in the MAC layer.

Radio Resource Allocation for Proximity Services

From the perspective of a transmitting UE, a Proximity-Services-enabledUE (ProSe-enabled UE) can operate in two modes for resource allocation.

Mode 1 refers to the eNB-scheduled resource allocation, where the UErequests transmission resources from the eNB (or Release-10 relay node),and the eNodeB (or Release-10 relay node) in turn schedules theresources used by a UE to transmit direct data and direct controlinformation (e.g., Scheduling Assignment). The UE needs to beRRC_CONNECTED in order to transmit data. In particular, the UE sends ascheduling request (D-SR or Random Access) to the eNB followed by abuffer status report (BSR) in the usual manner (see also followingchapter “Transmission procedure for D2D communication”). Based on theBSR, the eNB can determine that the UE has data for a ProSe DirectCommunication transmission and can estimate the resources needed fortransmission.

On the other hand, Mode 2 refers to the UE-autonomous resourceselection, where a UE on its own selects resources (time and frequency)from resource pool(s) to transmit direct data and direct controlinformation (i.e., SA). One resource pool is defined, e.g., by thecontent of SIB18, namely by the field commTxPoolNormalCommon, thisparticular resource pool being broadcast in the cell and then commonlyavailable for all UEs in the cell still in RRC_Idle state. Effectively,the eNB may define up to four different instances of said pool,respectively four resource pools for the transmission of SA messages anddirect data. However, a UE shall always use the first resource pooldefined in the list, even if it was configured with multiple resourcepools.

As an alternative, another resource pool can be defined by the eNB andsignaled in SIB18, namely by using the field commTxPoolExceptional,which can be used by the UEs in exceptional cases.

What resource allocation mode a UE is going to use is configurable bythe eNB. Furthermore, what resource allocation mode a UE is going to usefor D2D data communication may also depend on the RRC state, i.e.,RRC_IDLE or RRC_CONNECTED, and the coverage state of the UE, i.e.,in-coverage, out-of-coverage. A UE is considered in-coverage if it has aserving cell (i.e., the UE is RRC_CONNECTED or is camping on a cell inRRC_IDLE).

FIG. 4 illustrates the use of transmission/reception resources foroverlay (LTE) and underlay (D2D) system.

Basically, the eNodeB controls whether UE may apply the Mode 1 or Mode 2transmission. Once the UE knows its resources where it can transmit (orreceive) D2D communication, it uses the corresponding resources only forthe corresponding transmission/reception. For example, in FIG. 4 the D2Dsubframes will only be used to receive or transmit the D2D signals.Since the UE as a D2D device would operate in Half Duplex mode, it caneither receive or transmit the D2D signals at any point of time.Similarly, the other subframes illustrated in FIG. 4 can be used for LTE(overlay) transmissions and/or reception.

Transmission Procedure for D2D Communication

The D2D data transmission procedure differs depending on the resourceallocation mode. As described above for Mode 1, the eNB explicitlyschedules the resources for the Scheduling Assignment and the D2D datacommunication after a corresponding request from the UE. Particularly,the UE may be informed by the eNB that D2D communication is generallyallowed, but that no Mode 2 resources (i.e., resource pool) areprovided; this may be done, e.g., with the exchange of the D2Dcommunication Interest Indication by the UE and the correspondingresponse, D2D Communication Response, where the corresponding exemplaryProseCommConfig information element mentioned above would not includethe commTxPoolNormalCommon, meaning that a UE that wants to start directcommunication involving transmissions has to request E-UTRAN to assignresources for each individual transmission. Thus, in such a case, the UEhas to request the resources for each individual transmission, and inthe following the different steps of the request/grant procedure areexemplarily listed for this Mode 1 resource allocation:

-   -   Step 1: UE sends SR (Scheduling Request) to eNB via PUCCH;    -   Step 2: eNB grants UL resource (for UE to send BSR) via PDCCH,        scrambled by C-RNTI;    -   Step 3: UE sends D2D BSR indicating the buffer status via PUSCH;    -   Step 4: eNB grants D2D resource (for UE to send data) via PDCCH,        scrambled by D2D-RNTI.    -   Step 5: D2D Tx UE transmits SA/D2D data according to grant        received in step 4.

A Scheduling Assignment (SA), also termed SCI (Sidelink ControlInformation) is a compact (low-payload) message containing controlinformation, e.g., pointer(s) to time-frequency resources, modulationand coding scheme and Group Destination ID for the corresponding D2Ddata transmission. An SCI transports the sidelink scheduling informationfor one (ProSE) destination ID. The content of the SA (SCI) is basicallyin accordance with the grant received in Step 4 above. The D2D grant andSA content (i.e., SCI content) are defined in the 3GPP technicalstandard 36.212, current version 12.4.0, subclause 5.4.3, incorporatedherein by reference, defining in particular the SCI format 0.

On the other hand, for Mode 2 resource allocation, above steps 1-4 arebasically not necessary, and the UE autonomously selects resources forthe SA and D2D data transmission from the transmission resource pool(s)configured and provided by the eNB.

FIG. 5 exemplarily illustrates the transmission of the SchedulingAssignment and the D2D data for two UEs, UE-A and UE-B, where theresources for sending the scheduling assignments are periodic, and theresources used for the D2D data transmission are indicated by thecorresponding Scheduling Assignment.

ProSe Network Architecture and ProSe Entities

FIG. 6 illustrates a high-level exemplary architecture for a non-roamingcase, including different ProSe applications in the respective UEs A andB, as well as a ProSe Application Server and ProSe function in thenetwork. The example architecture of FIG. 6 is taken from 3GPP TS 23.303v.13.0.0 chapter 4.2 “Architectural Reference Model” incorporated hereinby reference.

The functional entities are presented and explained in detail in TS23.303 subclause 4.4 “Functional Entities” incorporated herein byreference. The ProSe function is the logical function that is used fornetwork-related actions required for ProSe and plays different roles foreach of the features of ProSe. The ProSe function is part of the 3GPP'sEPC and provides all relevant network services like authorization,authentication, data handling, etc., related to proximity services. ForProSe direct discovery and communication, the UE may obtain a specificProSe UE identity, other configuration information, as well asauthorization from the ProSe function over the PC3 reference point.There can be multiple ProSe functions deployed in the network, althoughfor ease of illustration a single ProSe function is presented. The ProSefunction consists of three main sub-functions that perform differentroles depending on the ProSe feature: Direct Provision Function (DPF),Direct Discovery Name Management Function, and EPC-level DiscoveryFunction. The DPF is used to provision the UE with the necessaryparameters to use ProSe Direct Discovery and ProSe Direct Communication.

The term “UE” used in said connection refers to a ProSe-enabled UEsupporting ProSe functionality.

The ProSe Application Server supports the Storage of EPC ProSe User IDs,and ProSe Function IDs, and the mapping of Application Layer User IDsand EPC ProSe User IDs. The ProSe Application Server (AS) is an entityoutside the scope of 3GPP. The ProSe application in the UE communicateswith the ProSe AS via the application-layer reference point PC1. TheProSe AS is connected to the 3GPP network via the PC2 reference point.

UE Coverage States for D2D

As already mentioned before, the resource allocation method for D2Dcommunication depends apart from the RRC state, i.e., RRC_IDLE andRRC_CONNECTED, also on the coverage state of the UE, i.e., in-coverage,out-of-coverage. A UE is considered in-coverage if it has a serving cell(i.e., the UE is RRC_CONNECTED or is camping on a cell in RRC_IDLE).

The two coverage states mentioned so far, i.e., in-coverage (IC) andout-of-coverage (OOC), are further distinguished into sub-states forD2D. FIG. 7 shows the four different states a D2D UE can be associatedto, which can be summarized as follows:

-   -   State 1: UE1 has uplink and downlink coverage. In this state the        network controls each D2D communication session. Furthermore,        the network configures whether UE1 should use resource        allocation Mode 1 or Mode 2.    -   State 2: UE2 has downlink but no uplink coverage, i.e., only DL        coverage. The network broadcasts a (contention-based) resource        pool. In this state the transmitting UE selects the resources        used for SA and data from a resource pool configured by the        network; resource allocation is only possible according to Mode        2 for D2D communication in such a state.    -   State 3: Since UE3 has no uplink and downlink coverage, the UE3        is, strictly speaking, already considered as out-of-coverage        (OOC). However, UE3 is in the coverage of some UEs which are        themselves (e.g., UE1) in the coverage of the cell, i.e., those        UEs can be also referred as CP-relay UEs or simply relay UEs        (see also later chapters on ProSe relay). Therefore, the area of        the state-3 UEs in FIG. 7 can be denoted as CP UE-relay coverage        area. UEs in this state 3 are also referred to as OOC-state-3        UEs. In this state the UEs may receive some cell-specific        information which is sent by the eNB (SIB) and forwarded by the        CP UE-relay UEs in the coverage of the cell via PD2DSCH to the        OOC-state-3 UEs. A (contention-based) network-controlled        resource pool is signaled by PD2DSCH.    -   State 4: UE4 is out of coverage and does not receive PD2DSCH        from other UEs which are in the coverage of a cell. In this        state, which is also referred to as state-4 OOC, the        transmitting UE selects the resources used for the data        transmission from a pre-configured pool of resources.

The reason to distinguish between state-3 OOC and state-4 OOC is mainlyto avoid potentially strong interference between D2D transmissions fromout-of-coverage devices and legacy E-UTRA transmissions. In general,D2D-capable UEs will have preconfigured resource pool(s) fortransmission of D2D SAs and data for use while out of coverage. If theseout-of-coverage UEs transmit on these preconfigured resource pools nearcell boundaries, then, interference between the D2D transmissions andin-coverage legacy transmissions could have a negative impact oncommunications within the cell. If D2D-enabled UEs within coverageforwarded the D2D resource pool configuration to those out-of-coveragedevices near the cell boundary, then, the out-of-coverage UEs couldrestrict their transmissions to the resources specified by the eNode Band therefore minimize interference with legacy transmissions incoverage. Thus, RANI introduced a mechanism where in-coverage UEs areforwarding resource pool information and other D2D relatedconfigurations to those devices just outside the coverage area (state-3UEs).

The Physical D2D synchronization channel (PD2DSCH) is used to carry thisinformation about in-coverage D2D resource pools to the UEs in networkproximity, so that resource pools within network proximity are aligned.

D2D Discovery

ProSe Direct Discovery is defined as the procedure used by theProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximityusing E-UTRA direct radio signals via PC5. FIG. 8 schematicallyillustrates the PC5 interface for device-to-device direct discovery.

Upper layer handles authorization for announcement and monitoring ofdiscovery information. For this purpose, UEs have to exchange predefinedsignals, referred to as “discovery signals”. By checking discoverysignals periodically, a UE maintains a list of proximity UEs in order toestablish a communication link when needed. Discovery signals should bedetected reliably, even in low Signal-to-Noise Ratio (SNR) environments.To allow discovery signals to be transmitted periodically, resources forDiscovery signals should be assigned.

There are two types of ProSe Direct Discovery: open and restricted. Openis the case where there is no explicit permission that is needed fromthe UE being discovered, whereas restricted discovery only takes placewith explicit permission from the UE that is being discovered.

ProSe Direct Discovery can be a standalone service enabler that couldfor example use information from the discovered UE for certainapplications in the UE that are permitted to use this information, e.g.,“find a taxi nearby”, “find me a coffee shop”. Additionally depending onthe information obtained, ProSe Direct Discovery can be used forsubsequent actions, e.g., to initiate ProSe Direct Communication.

ProSe Direct Discovery Models

The following models for ProSe Direct Discovery are defined in thestandard 3GPP TS 23.303, current version 13.0.0, section 5.3 and allsubsections thereof, incorporated herein by reference.

Model A (“I am here”):

This model defines two roles for the ProSe-enabled UEs that areparticipating in ProSe Direct Discovery.

Announcing UE: The UE announces certain information that could be usedby UEs in proximity that have permission to discover.

Monitoring UE: The UE that monitors certain information of interest inproximity of announcing UEs.

In this model the announcing UE broadcasts discovery messages atpre-defined discovery intervals and the monitoring UEs that areinterested in these messages read them and process them. This model maybe referred to as “I am here” since the announcing UE would broadcastinformation about itself, e.g., its ProSe Application Code in thediscovery message.

Model B (“who is there?”/“are you there?”):

This model defines two roles for the ProSe-enabled UEs that areparticipating in ProSe Direct Discovery.

-   -   Discoverer UE: The UE transmits a request containing certain        information about what it is interested to discover.    -   Discoveree UE: The UE that receives the request message can        respond with some information related to the discoverer's        request.

It can be referred to as “who is there/are you there” since thediscoverer UE sends information for other UEs that would like to receiveresponses, e.g., the information can be about a ProSe ApplicationIdentity corresponding to a group and the members of the group canrespond.

The content of the discovery information is transparent to the AccessStratum (AS), and no distinction is made in the AS for ProSe DirectDiscovery models and types of ProSe Direct Discovery. The ProSe Protocolensures that it delivers only valid discovery information to AS forannouncement.

The UE can participate in announcing and monitoring of discoveryinformation in both RRC_IDLE and RRC_CONNECTED state as per eNBconfiguration. The UE announces and monitors its discovery informationsubject to the half-duplex constraints.

Resource Allocation for Discovery

D2D communication may either be network-controlled where the operatormanages the switching between direct transmissions (D2D) andconventional cellular links, or the direct links may be managed by thedevices without operator control. D2D allows combininginfrastructure-mode and ad hoc communication.

Generally, device discovery is needed periodically. Further, D2D devicesutilize a discovery message signaling protocol to perform devicediscovery. For example, a D2D-enabled UE can transmit its discoverymessage, and another D2D-enabled UE receives this discovery message andcan use the information to establish a direct communication link. Anadvantage of a hybrid network is that if D2D devices are also incommunication range of network infrastructure, network entities, likeeNB, can additionally assist in the transmission or configuration ofdiscovery messages. Coordination/control by the eNB in the transmissionor configuration of discovery messages is also important to ensure thatD2D messaging does not create interference with the cellular trafficcontrolled by the eNB. Additionally, even if some of the devices areoutside of the network coverage range, in-coverage devices can assist inthe ad-hoc discovery protocol.

At least the following two types of discovery procedure are defined forthe purpose of terminology definition used further in the description.

-   -   UE autonomous resource selection (called Type 1 subsequently): A        resource allocation procedure where resources for announcing        discovery information are allocated on a non UE specific basis,        further characterized by:        -   The eNB provides the UE(s) with the resource pool            configuration used for announcing of discovery information.            The configuration may be, e.g., signaled in SIB.        -   The UE autonomously selects radio resource(s) from the            indicated resource pool and announces discovery information.        -   The UE can announce discovery information on a randomly            selected discovery resource during each discovery period.    -   Scheduled resource allocation (called Type 2 subsequently): A        resource allocation procedure where resources for announcing        discovery information are allocated on a per-UE-specific basis,        further characterized by:        -   The UE in RRC_CONNECTED may request resource(s) for            announcing of discovery information from the eNB via RRC.            The eNB assigns resource(s) via RRC.        -   The resources are allocated within the resource pool that is            configured in UEs for monitoring.

For UEs in RRC_IDLE the eNB may select one of the following options:

-   -   The eNB may provide a Type 1 resource pool for discovery        information announcement in SIB. UEs that are authorized for        Prose Direct Discovery use these resources for announcing        discovery information in RRC_IDLE.    -   The eNB may indicate in SIB that it supports D2D but does not        provide resources for discovery information announcement. UEs        need to enter RRC Connected in order to request D2D resources        for discovery information announcement.

For UEs in RRC_CONNECTED:

-   -   A UE authorized to perform ProSe Direct Discovery announcement        indicates to the eNB that it wants to perform D2D discovery        announcement    -   The eNB validates whether the UE is authorized for ProSe Direct        Discovery announcement using the UE context received from MME.    -   The eNB may configure the UE to use a Type 1 resource pool or        dedicated Type 2 resources for discovery information        announcement via dedicated RRC signaling (or no resource).    -   The resources allocated by the eNB are valid until a) the eNB        de-configures the resource (s) by RRC signaling or b) the UE        enters IDLE.

Receiving UEs in RRC_IDLE and RRC_CONNECTED monitor both Type 1 and Type2 discovery resource pools as authorized. The eNB provides the resourcepool configuration used for discovery information monitoring in SIB. TheSIB may contain discovery resources used for announcing in neighborcells as well.

Radio Protocol Architecture for ProSe Direct Discovery

FIG. 9 schematically illustrates a Radio Protocol Stack (Access Stratum)for ProSe Direct Discovery, where the access stratum protocol consistsof only MAC and PHY. The AS layer performs the following functions:

-   -   Interfaces with upper layer (ProSe Protocol): The MAC layer        receives the discovery message from the upper layer (ProSe        Protocol). The IP layer is not used for transmitting the        discovery message;    -   Scheduling: The MAC layer determines the radio resource to be        used for announcing the discovery message received from upper        layer;    -   Discovery PDU generation: The MAC layer builds the MAC PDU        carrying the discovery message and sends the MAC PDU to the        physical layer for transmission in the determined radio        resource. No MAC header is added.

In the UE, the RRC protocol informs the discovery resource pools to MAC.RRC also informs allocated Type 2B resource for transmission to MAC.There is no need for a MAC header. MAC header for discovery does notcomprise any fields based on which filtering on L2 could be performed.Discovery message filtering at the MAC level does not seem to saveprocessing or power compared to performing filtering at the upper layersbased on the Prose UE- and/or Prose Application ID. The MAC receiverforwards all received discovery messages to upper layers. MAC willdeliver only correctly received messages to upper layers. It is assumedthat L1 indicates to MAC whether a discovery messages has been receivedcorrectly. It is assumed that Upper Layers guarantee to deliver onlyvalid discovery information to the Access Stratum.

ProSe UE-to-Network Relay

A UE may also support the functionality and procedure(s) so as to act asa ProSe UE-to-Network Relay, such that a Remote UE communicates with theProSe UE-to-Network Relay over the PC5 reference point. ProSeUE-to-Network Relay operation will be specified within 3GPP Release 13.So far, only initial agreements have been made in the 3GPP RAN workinggroups, some of which can be seen, e.g., from 3GPP TS 23.303 currentversion 13.0.0 and 3GPP TR 23.713 current version 1.4.0, incorporatedherein by reference. Some of those agreements will be listed below. Itshould however be noted that this work item has been introduced veryrecently and thus is still in the process of standardization.Consequently, any agreements assumed in the following can still bechanged or reversed, and the following agreements, which are assumed fordiscussion purposes, shall however not be understood as limiting thepresent disclosure to this particular 3GPP implementation at this veryearly stage of standardization.

-   -   For the ProSe UE-to-Network Relay discovery and ProSe relay        (re)selection both scenarios where Remote UEs are in-coverage        and out-of-coverage can be addressed.    -   Relay UE will always be in-coverage. The eNB at the radio level        can control whether the UE can act as a relay, whereas whether        the network control is per relay UE, per cell (broadcast        configuration), or both, or something else is still undecided.    -   When Remote UE is in-coverage for relay discovery purposes, the        monitoring and transmitting resources for discovery can be        provided, e.g., by the eNB using the Rel-12 mechanisms        (broadcast for idle mode and dedicated signaling for connected        mode). The remote UE can decide when to start monitoring.    -   When the Remote UE is out of coverage, the monitoring and        transmitting resources for discovery and communication (actual        data transfer) can be provided, e.g., by pre-configuration i.e.,        by way of specification/operator configuration (in USIM, etc.)        such that the UE exactly knows which resources to use.

ProSe UE-to-Network Relay (re)selection:

-   -   The Remote UE can take radio level measurements of the PC5 radio        link quality into account for the ProSe UE-to-Network Relay        selection procedure.    -   For the case that the Remote UE is out-of-coverage, the radio        level measurements can be used by the remote UE together with        other higher layer criteria to perform relay selection.    -   For the case that Remote UE is out-of-coverage, the criteria for        reselection is based on PC5 measurements (RSRP or other RANI        agreed measurements) and higher layer criteria. The relay        reselection can be triggered by the remote UE.    -   For the case that the Remote UE is in-coverage, it is not yet        decided whether and how these measurements (PC5 measurements)        are used (e.g., the measurements can be used by the UE to        perform selection similar to out-of-coverage case, or they can        be reported to the eNB).

The ProSe UE-to-Network relay may use layer-3 packet forwarding. Controlinformation between ProSe UEs can be exchanged over the PC5 referencepoint, e.g., for UE-to-Network Relay detection and ProSe DirectDiscovery.

A ProSe-enabled UE will also support the exchange of ProSe controlinformation between another ProSe-enabled UE and the ProSe Function overthe PC3 reference point. In the ProSe UE-to-Network Relay case, theRemote UE will send this control information over the PC5 user plane tobe relayed over the LTE-Uu interface towards the ProSe Function.

The ProSe UE-to-Network Relay entity provides the functionality tosupport connectivity to “unicast” services for Remote UEs that are notin the coverage area of an eNB, i.e., not connected to E-UTRAN. FIG. 10shows a ProSe UE-to-Network Relay scenario. The ProSe UE-to-NetworkRelay shall relay unicast traffic (UL and/or DL) between the Remote UEand the network. The ProSe UE-to-Network Relay shall provide a genericfunction that can relay any type of traffic that is relevant for publicsafety communication.

One-to-one Direct Communication between Remote UEs and ProSeUE-to-Network Relays has the following characteristics:

-   -   Communication over PC5 reference point is connectionless.    -   ProSe Bearers are bi-directional. IP packets passed to the radio        layers on a given ProSe bearer will be transmitted by the        physical layer with the associated L2 destination address. IP        packets passed up from the radio layers on the same ProSe bearer        will have been received over the air addressed to the same L2        destination.

ProSe UE-to-Network Relaying may include the following functions:

-   -   ProSe Direct discovery following Model A or Model B can be used        in order to allow the Remote UE to discover ProSe UE-to-Network        Relay(s) in proximity.    -   ProSe Direct discovery that can be used in order to allow the        Remote UE to discover L2 address of the ProSe UE-to-Network        Relay to be used by the Remote UE for IP address allocation and        user plane traffic corresponding to a specific PDN connection        supported by the ProSe UE-to-Network Relay.    -   Act as an “announcing” or “discoveree” UE on the PC5 reference        point supporting direct discovery.    -   Act as a default router to the Remote UEs forwarding IP packets        between the UE-ProSe UE-to-Network Relay point-to-point link and        the corresponding PDN connection.    -   Handle Router Solicitation and Router Advertisement messages as        defined in IETF RFC 4861.    -   Act as DHCPv4 Server and stateless DHCPv6 Relay Agent.    -   Act as a NAT if IPv4 is used replacing the locally assigned IPv4        address of the Remote UE with its own.    -   Map the L2 link ID used by the Remote UE as Destination Layer-2        ID to the corresponding PDN connection supported by the ProSe        UE-to-Network Relay.

The user plane protocol architecture for the ProSe UE-to-Network relayis shown in FIG. 11.

ProSe UE-to-Network Relay Discovery

Both Model A and Model B discovery are supported, as discussed beforefor the usual Rel.-12 direct discovery between two ProSe UEs, whereModel A uses a single discovery protocol message (UE-to-Network RelayDiscovery Announcement) and Model B uses two discovery protocol messages(UE-to-Network Relay Discovery Solicitation and UE-to-Network RelayDiscovery Response). Details on Relay Discovery can be found in section6 of 3GPP TR 23.713 current version v1.4.0 incorporated herein byreference.

The following parameters are common to all of UE-to-Network RelayDiscovery, Group Member Discovery and UE-to-UE Relay Discovery:

-   -   Message type: Announcement (Model A) or Solicitation/Response        (Model B), Relay Discovery Additional Information (Model A).    -   Discovery type: indicates whether this is UE-to-Network Relay        Discovery, Group Member Discovery or UE-to-UE Relay Discovery.

The following parameters are used in the UE-to-Network Relay DiscoveryAnnouncement message (Model A):

-   -   ProSe Relay UE ID: link layer identifier that is used for direct        communication and is associated with a PDN connection the ProSe        UE-to-Network Relay has established.    -   Announcer info: provides information about the announcing user.    -   Relay Service Code: parameter identifying a connectivity service        the ProSe UE-to-Network Relay provides to Public Safety        applications. The Relay Service Codes are configured in a ProSe        UE-to-Network relay for advertisement and map in the ProSe        UE-to-Network relay to specific APNs they offer connectivity to.        Additionally, the Relay Service Code also identifies authorized        users the ProSe UE-to-Network relay would offer service to, and        may select the related security policies or information, e.g.,        necessary for authentication and authorization between the        Remote UE and the ProSe UE-to-Network Relay (e.g., a Relay        Service Code for relays for police members only would be        different than a Relay Service code for relays for Fire Fighters        only, even though potentially they provided connectivity to same        APN, e.g., to support Internet Access).    -   Radio Layer Information: contains information about the radio        layer information, e.g., radio conditions between the eNB and        the UE-to-Network Relay, to assist the Remote UE selecting the        proper UE-to-Network Relay.

The following parameters are used in the UE-to-Network Relay DiscoverySolicitation message (Model B):

-   -   Discoverer info: provides information about the discoverer user.    -   Relay Service Code: information about connectivity that the        discoverer UE is interested in. The Relay Service Codes are        configured in the Prose Remote UEs interested in related        connectivity services.    -   ProSe UE ID: link layer identifier of the discoverer that is        used for direct communication (Model B).

The following parameters are used in the UE-to-Network Relay DiscoveryResponse message (Model B):

-   -   ProSe Relay UE ID: link layer identifier that is used for direct        communication and is associated with a PDN connection the ProSe        UE-to-Network Relay has established.    -   Discoveree info: provides information about the discoveree.    -   Radio Layer Information: contains information about the radio        layer information, e.g., radio conditions between the eNB and        the UE-to-Network Relay, to assist the Remote UE selecting the        proper UE-to-Network Relay.

ProSe Direction Communication Via the ProSe UE-to-Network Relay

The UE-to-Network Relay function will be specified based upon anevolution of the ProSe functionality already documented in TS 23.303.

A ProSe UE-to-Network Relay capable UE may attach to the network (if itis not already connected) and connect to a PDN connection enabling thenecessary relay traffic, or it may need to connect to additional PDNconnection(s) in order to provide relay traffic towards Remote UE(s).PDN connection(s) supporting UE-to-Network Relay shall only be used forRemote ProSe UE(s) relay traffic. FIG. 12 illustrates the directcommunication via a ProSe UE-to-Network Relay.

1. The ProSe UE-Network Relay performs initial E-UTRAN Attach (if notalready attached) and/or establishes a PDN connection for relaying (ifno appropriate PDN connection for this relaying exists yet). In case ofIPv6, the ProSe UE-Network Relay obtains an IPv6 prefix via prefixdelegation function from the network as defined in TS 23.401.

2. The Remote UE performs discovery of a ProSe UE-Network Relay usingModel A or Model B discovery. The details of this procedure weredescribed before.

3. The Remote UE uses the received relay selection information to selecta ProSe UE-Network Relay and then establishes a connection forOne-to-One Communication as discussed before with reference to FIG. 3.

4. When IPv6 is used on PCS, the Remote UE performs IPv6 StatelessAddress auto-configuration, where the Remote UE shall send a RouterSolicitation message (step 4a) to the network using as DestinationLayer-2 ID the Layer-2 ID of the Relay in order to solicit a RouterAdvertisement message (step 4b) as specified in IETF RFC 4862. TheRouter Advertisement messages shall contain the assigned IPv6 prefix.After the Remote UE receives the Router Advertisement message, itconstructs a full IPv6 address via IPv6 Stateless Addressauto-configuration in accordance with IETF RFC 4862. However, the RemoteUE shall not use any identifiers defined in TS 23.003 as the basis forgenerating the interface identifier. For privacy, the Remote UE maychange the interface identifier used to generate the full IPv6 address,as defined in TS 23.221 without involving the network. The Remote UEshall use the auto-configured IPv6 address while sending packets.

5. When IPv4 is used on PC5, the Remote UE uses DHCPv4. The Remote UEshall send DHCPv4 Discovery (step 5a) message using as DestinationLayer-2 ID the Layer-2 ID of the Relay. The ProSe UE-Network Relayacting as a DHCPv4 Server sends the DHCPv4 Offer (step 5b) with theassigned Remote UE IPv4 address. When the Remote UE receives the leaseoffer, it sends a DHCP REQUEST message containing the received IPv4address (step 5c). The ProSe UE-Network Relay acting as DHCPv4 serversends a DHCPACK message to the Remote UE (step 5d) including the leaseduration and any other configuration information that the client mighthave requested. On receiving the DHCPACK message, the Remote UEcompletes the TCP/IP configuration process.

As has been explained above, 3GPP introduces as a major work item theProSe relay functionality, which includes relay discovery and relaydirect communication. Some of the currently-defined mechanisms for ProSerelay are rather inefficient. Other mechanisms are not agreed at all,such as how and when a relay-capable ProSe UE actually starts acting asa relay, i.e., offering the relay service in the radio cell.

BRIEF SUMMARY

One non-limiting and exemplary embodiment provides improved methods foractivating the relay functionality of a relay user equipment. Theindependent claims provide non-limiting and exemplary embodiments.Advantageous embodiments are subject to the dependent claims.

According to several aspects described herein, the activation of therelay functionality in a relay-capable user equipment is improved. Inorder to discuss these aspects the following assumptions are made. Inparticular, it is assumed that the relevant relay user equipment iscapable of performing direct communications with other remote userequipment(s) (i.e., via a direct sidelink connection). Furthermore, theterm “relay-capable user equipment” shall be understood in that the userequipment supports a relay functionality for being capable of serving asa relay respectively for one or more remote user equipments, whicheventually entails performing a relay discovery procedure and, uponbeing selected by a remote user equipment, establishing a directsidelink connection via which said remote user equipment is connected.Communication between the one or more remote user equipments and a radiobase station (to which the relay user equipment is connected) is relayedby the relay user equipment via the established direct sidelinkconnection.

According to a first aspect, the activation of the relay functionalityof a relay user equipment is controlled as follows. The radio basestation first determines whether further relays are actually necessaryin the radio cell. For example, in order to be able to determine whetherfurther relays are necessary, the radio base station may monitor thenumber of relay user equipments in the radio cell with an activatedrelay functionality, and/or the number of remote user equipments in theradio cell which might need to be served by a relay so as to keep beingconnected to the radio base station and thus to be able to further usethe services provided by the communication network (e.g., core and radionetwork). For the determination, the radio base station may further takeinto account the number of remote user equipments that are runningpublic safety services in the radio cell. Alternatively, a ProSefunction may determine whether further relays are actually necessary inthe radio cell of the radio base station and may thus provide acorresponding indication to the radio base station, which in turn, basedon this indication, determines that further relays are actuallynecessary and continues with the relay activation.

As a result, when the radio base station confirms that further relaysare necessary in the radio cell, it will broadcast a correspondingbroadcast message in its radio cell so is to indicate this, i.e., thatfurther relays are necessary in the radio cell and that therelay-capable user equipments (which have not yet activated their relayfunctionality) should start the relay activation procedure describedbelow. In other words, the broadcast message from the radio base stationcan be seen as the relay activation trigger for the corresponding relayuser equipments (i.e., those relay-capable user equipments that have noactivated relay functionality) to start the relay activation procedurein the relay user equipment to determine whether to activate the relayfunctionality or not.

Furthermore, the radio base station will select a persistence checkvalue based on the number of further relays that are needed in the radiocell. In particular, the relay activation procedure to be performed inthe relay user equipment(s) comprises performing a correspondingpersistence check (including generating a random value to be compared tothe persistence check value) so as to limit the total number of relayuser equipments that will actually activate the relay functionality inresponse to the trigger. By setting the persistence check value to anappropriate value, the radio base station has implicit control over themaximum number of relay user equipments that might eventually activatethe relay functionality.

Furthermore, the relay activation procedure to be performed in the relayuser equipments includes a further check, namely whether or not specificrelay requirements defined in the radio cell for activating the relayfunctionality are fulfilled or not by the relay user equipment.

According to this first aspect, the relay user equipment is allowed toactivate its relay functionality only if both the persistence check wassuccessful and the relay user equipment fulfils all the necessary relayrequirements defined for the radio cell. After the relay user equipmentactivates its relay functionality, it may start with the relay discoveryprocedure which involves transmitting relay discovery messages in theradio cell so as to announce its presence as a relay in the radio celland thus allows the activated relay user equipment to be discovered byother remote user equipments; model A or model B discovery can beperformed. Eventually, if the relay is then selected by the remote userequipment to act as a relay, a direct sidelink connection will beestablished between the relay user equipment and the remote userequipment over which the communication with the radio base station canbe relayed.

As discussed above, the relay activation procedure to be performed inthe relay user equipment according to the first aspect comprises twochecks, the persistence check based on the received persistence checkvalue (threshold) and the relay requirements check. It should be notedthat the order in which the two checks are performed is irrelevant tothe overall functioning of the first aspect. For instance, the twochecks may be performed in parallel, or subsequently, whereasadvantageously the second check shall only then be performed when thefirst (previous) check was successful.

A possible implementation of the persistence check performed in therelay user equipment according to the first aspect will be explainednow. The persistence check value was selected by the radio base stationfrom a range of values (e.g., between 0 and 1). Then, the relay userequipment generates a random value in this same range of values. Thepersistence check value provided by the radio base station can be seenas a threshold with which the generated random value is compared so asto determine whether the persistence check is successful or not. In saidrespect, one alternative would be to consider the persistence checksuccessful if the generated random value is smaller than or equal to thepersistence check value threshold. Alternatively of course, thepersistence check could be considered successful if they generatedrandom value is above the persistence check value threshold. In eithercase, by appropriately selecting the persistence check value(threshold), the radio base station controls the percentage ofsuccessful persistence checks performed by all the relay user equipments(that perform this persistence check).

An exemplary variant of the first aspect also considers whether suitableradio resources are available in the relay user equipment(s) so as toperform the relay discovery procedure that follows in case the relayfunctionality is indeed activated. In particular, the relay userequipment may additionally determine whether such radio resources arealready configured and usable to perform the relay discovery procedure.If no suitable radio resources are available to be used by the relayuser equipment, a corresponding request to the radio base station can beperformed by the relay user equipment, where the radio base station inturn may transmit a response message back, indicating whether and whichradio resources are assigned to the relay user equipment so as toperform the relay discovery procedure. On the other hand, in theaffirmative case, no such request is necessary. Advantageously, thisdetermination on the available radio resources can be performed afterthe two checks of the relay activation procedure (i.e., the persistencecheck and relay requirements check) are finished successfully to therebyavoid to unnecessarily request the radio resources from the radio basestation.

According to a further variant of the first aspect, the relay userequipment might still have to seek permission from the radio basestation as to whether the relay functionality is allowed to be activatedor not; even when the two checks of the relay activation procedure arefinished successfully. In other words, after the two checks of the relayactivation procedure are finished successfully by the relay userequipment, the relay user equipment shall transmit a relay activationrequest message to the radio base station so as to request permissionfrom the radio base station to activate the relay functionality.Correspondingly, the radio base station will decide on whether to grantor deny the permission for each of the requesting relay user equipments,and will eventually transmit a corresponding relay activation responsemessage back to the relay user equipment, giving or denying permissionto activate the relay functionality in the relay user equipment.Therefore, the activation of the relay functionality in each of therelay user equipment(s) will be ultimately controlled by the radio basestation.

In a further improvement of this variant of the first aspect, therequesting of resources for the relay discovery procedure can becombined with the step of seeking permission from the radio base stationto activate the relay functionality. In particular, these two steps maybe performed at the same time, e.g., by transmitting the relayactivation request message which not only shall seek permission toactivate the relay functionality but which shall also request radioresources for a possible relay discovery procedure to be performedafterwards. Likewise, the relay activation response message, transmittedfrom the radio base station, may not only include the permission ordenial to activate the relay functionality, but may also include theradio resources to be used for the relay discovery procedure (to beperformed in case the relay functionality is indeed activated).Advantageously, the granting of radio resources, i.e., the presence ofcorresponding information on the granted radio resources in the relayactivation response message, can already be interpreted by the receivingrelay user equipment as the permission to activate its relayfunctionality (i.e., denying the permission to activate the relayfunctionality and at the same time granting radio resources for therelay discovery procedure, is disadvantageous). In a similar manner,although it might be possible to have the request for permissionseparately from the request of radio resources in the relay activationrequest message, the request of radio resources for the relay discoveryprocedure might already be interpreted by the radio base station as therelay user equipment seeking permission to activate its relayfunctionality.

Further variants of the first aspect distinguish between relay userequipments in connected state and relay user equipments in idle state.The relay user equipment determines in which state it is, and in case itis in the idle state, a transition to the connected state may beperformed by the relay user equipment. There may be several reasons whyit is advantageous for the relay user equipment to transition to theconnected state. One particular reason is that, although relay discoverymay be performed also when being in idle state, the activated relayfunctionality, i.e., serving as a relay for a remote UE, requires therelay user equipment to be in connected state so as to allow therelaying of the communication with the radio base station. Furthermore,when assuming the previous variants where radio resources might have tobe first requested from the radio base station and/or where the radiobase station has to be sought for permission to activate the relayfunctionality, the transmission of such request(s) can only be performedby the relay user equipment when being in a connected state (i.e., whenhaving an active connection with the radio base station). It should benoted that the transition to the connected state may involve performinga connection request procedure such that a connection with the radiobase station is established. In said respect it may be advantageous toalso indicate as the establishment cause that the transition is causedby the relay activation procedure (i.e., the need to request radioresources to perform the relay discovery procedure and/or the need toseek permission from the radio base station to activate the relayfunctionality).

In further variants of the first aspect, the broadcast message may beextended by including the additional relay requirements that are to befulfilled by prospective relay user equipments before being allowed toactivate their relay functionality. Although the indication that furtherrelays are necessary may be separately provided in the broadcast messagefrom the information on the relay requirements, advantageously, thereception of the broadcast message comprising the additional relayrequirements might be implicitly interpreted by the relay userequipment(s) as the indication that further relays are actuallynecessary in the radio cell and thus as the trigger to start the relayactivation procedure in the relay user equipment.

In addition or alternatively, the broadcast message may be extended byincluding information on radio resources to be used by the relay userequipment for performing the relay discovery procedure after havingactivated the relay functionality. Again, although the indication thatfurther relays are necessary may be separately provided in the broadcastmessage from the information on the radio resources to be used for therelay discovery procedure, advantageously, the reception of thebroadcast message comprising the information on the radio resources forthe relay discovery procedure may be implicitly interpreted by the relayuser equipment(s) as the indication that further relays are actuallynecessary in the radio cell and thus as the trigger to start the relayactivation procedure in the relay user equipment.

As discussed above, the relay activation procedure comprises the stepperformed by the relay user equipment of determining whether the relayuser equipment fulfills the additional relay requirements. Exemplarily,the relay requirements that may be one of the following: a minimumand/or maximum threshold for a radio link quality of a link between therelay user equipment and the radio base station, such as the referencesignal receive power, RSRP, or the reference signal received quality,RSRQ, a maximum threshold for a movement level of the relay userequipment, and a minimum threshold for a battery level of the relay userequipment.

In one general aspect, the techniques disclosed here feature a methodfor activating a relay functionality of a relay user equipment within amobile communication network. The relay user equipment is capable ofperforming direct communication over a direct sidelink connectionrespectively with one or more remote user equipments. The relay userequipment is located in a radio cell controlled by a radio base stationin the mobile communication network and supports a relay functionalityfor being capable of serving as a relay, respectively for the one ormore remote user equipments, so as to relay communication between theone or more remote user equipments and the radio base station via thedirect sidelink connection. The radio base station determines whether ornot further relays are necessary in the radio cell. In case it isdetermined that further relays are necessary in the radio cell, theradio base station selects a persistence check value and transmits abroadcast message in the radio cell. The broadcast message at leastindicates that further relays are necessary in the radio cell andcomprises the selected persistence check value. Upon receiving thebroadcast message, the relay user equipment activates its relayfunctionality in case the relay user equipment determines that relayrequirements for activating its relay functionality in the radio cellare fulfilled by the relay user equipment and in case a persistencecheck performed by the relay user equipment based on the receivedpersistence check value is successful.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows an exemplary architecture of a 3GPP LTE system;

FIG. 2 shows an exemplary downlink resource grid of a downlink slot of asubframe as defined for 3GPP LTE (Release 8/9);

FIG. 3 schematically illustrates how to establish a layer-2 link overthe PC5 for ProSe communication;

FIG. 4 illustrates the use of transmission/reception resources foroverlay (LTE) and underlay (D2D) systems;

FIG. 5 illustrates the transmission of the Scheduling Assignment and theD2D data for two UEs;

FIG. 6 illustrates an exemplary architecture model for ProSe for anon-roaming scenario;

FIG. 7 illustrates cell coverage regarding four different states the D2DUE can be associated to;

FIG. 8 schematically illustrates a PC5 interface for device-to-devicedirect discovery;

FIG. 9 schematically illustrates a radio protocol stack for ProSe directdiscovery;

FIG. 10 shows a ProSe UE-to-Network Relay scenario;

FIG. 11 shows the user plane protocol architecture for the ProSeUE-to-Network relay;

FIG. 12 illustrates the direct communication via ProSe UE-to-NetworkRelay for relay discovery and one-to-one communication establishment;

FIG. 13 is an alternative sequence diagram for the relay UE behavioraccording to exemplary implementations of the first embodiment;

FIG. 14 is an alternative sequence diagram for the relay UE behavioraccording to exemplary implementations of the first embodiment;

FIG. 15 is a sequence diagram for the relay UE behavior according to afurther implementation of the first embodiment where the relay UEadditionally needs to seek permission from the radio base station toactivate the relay functionality;

FIG. 16 is a sequence diagram for the relay UE behavior according to afurther implementation of the first embodiment where the relay UEadditionally determines whether radio resources are already availablefor the relay discovery procedure and react accordingly;

FIG. 17 is a sequence diagram for the relay UE behavior according tostill another implementation of the first embodiment;

FIG. 18 is a sequence diagram for the relay UE behavior according toanother implementation of the first embodiment, where the proceduretakes into account whether the relay UE is an idle or a connected stateand react accordingly;

FIG. 19 is a sequence diagram for the relay UE behavior according toanother implementation of the first embodiment, where a ProSe functionprovides a further level of control for initiating the relay activation;and

FIG. 20 is a sequence diagram for the relay UE behavior according toanother implementation of the first embodiment, where a ProSe functionadditionally is able to terminate the relay activation.

DETAILED DESCRIPTION

A mobile station or mobile node or user terminal or user equipment is aphysical entity within a communication network. One node may haveseveral functional entities. A functional entity refers to a software orhardware module that implements and/or offers a predetermined set offunctions to other functional entities of a node or the network. Nodesmay have one or more interfaces that attach the node to a communicationfacility or medium over which nodes can communicate. Similarly, anetwork entity may have a logical interface attaching the functionalentity to a communication facility or medium over which it maycommunicate with other functional entities or correspondent nodes.

A “relay user equipment” as used in the set of claims and in theapplication is to be broadly understood as referring to a user equipmentwhich is capable of serving as a relay for another user equipment(termed “remote user equipment”). This also involves the capability ofsupporting direct communication transmissions directly between two userequipments (see below D2D or ProSe). According to one implementation,the relay user equipment shall support relay functionality as definedfor 3GPP LTE-A and as described in the background section. In saidconnection, the term “remote user equipment” shall merely indicate therole of the user equipment as being the peer of the relay userequipment, i.e., looking for a relay to establish direct communicationwith.

The term “radio resources” as used in the set of claims and in theapplication is to be broadly understood as referring to physical radioresources, such as time-frequency resources.

The term “direct communication transmission” as used in the set ofclaims and in the application is to be broadly understood as atransmission directly between two user equipments, i.e., not via theradio base station (e.g., eNB). Correspondingly, the directcommunication transmission is performed over a “direct sidelinkconnection”, which is the term used for a connection establisheddirectly between two user equipments. For example, in 3GPP theterminology of D2D (Device-to-Device) communication is used or ProSecommunication, or a sidelink communication. The term “direct sidelinkconnection” as used in the set of claims and in the application is to bebroadly understood and can be understood in the 3GPP context as the PC5interface described in the background section.

The term “relay functionality” as used in the set of claims and in theapplication is to be broadly understood as the capability of a userequipment to act as a relay. In one exemplary implementation, the relayfunctionality is the functionality currently being standardized in the3GPP work item as explained in detail in the background section.

The term “persistence check” as used in the set of claims and in theapplication is to be broadly understood as a simple determination basedon comparing a randomly-generated value against a given threshold todetermine whether the persistence check is successful or not. Byappropriately selecting the threshold, it is possible to control(roughly) which percentage of the persistence checks will be successful.

3GPP is currently in the process of introducing a relay functionalityfor the ProSe-capable user equipments. Although some initial agreementshave been achieved already (some of which are explained in detail in thebackground section), no agreements could yet be achieved for someimportant issues in connection with the ProSe relay functionality. Oneimportant issue in said respect is the question of how and when a ProSerelay capable UE will actually start to be a relay UE, i.e., to activateits relay functionality so as to be able to serve as a relay for otherProSe remote UEs. It should be noted that a relay UE with an activatedrelay functionality will perform a relay discovery procedure so as toallow its discovery for remote UEs in its proximity, which comprises thetransmission of relay discovery messages according to model A (periodic)or model B (upon being solicited by a remote UE).

One possible way of controlling the relay activation for relay UEsinvolves the determination of whether or not the relay-capable UEfulfills particular pre-requisites to act as a relay in a particularradio cell. In more detail, it is assumed that, although a UE is ingeneral capable of acting as a relay, particular (relay) requirementsare defined in a radio cell (e.g., by the eNB) which are to beadditionally fulfilled before being allowed to act as a relay. Forinstance, the quality of the link between the relay UE and the eNodeBshould be good enough, i.e., higher than a minimum threshold, such thatit is guaranteed that the relay capable UEs will be able to serve as arelay relaying additional traffic coming from a remote UE. In anotherexample, also the speed with which a potential relay UE is moving shallbe limited to a particular maximum value since it is more likely that aremote UE connected to a fast-moving relay UE will soon get out of thetransmission range of the relay UE, thus having to select a new relay.Another possible relay requirement might refer to the battery level ofthe relay user equipment which should not fall below a certain minimumthreshold so as to guarantee service continuity for the remote UEselecting the particular relay UE to establish a connection with thenetwork.

It should be noted however that using relay requirements additionallydefined in a radio cell may have the disadvantage that too many(unnecessary) UEs will activate its relay functionality in a radio cell.As mentioned above, the activation of the relay functionality involvesthe start of the relay discovery procedure which in turn comprises thetransmission of relay discovery messages (e.g., in model A,periodically). This might unnecessarily increase the contention andinterference in the relay discovery resources and thus in turn may delaythe relay selection since the remote UE would need to try on more thanone potential relay device to receive connectivity subsequently oneafter the other until it succeeds in connecting to a relay. Even then,the communication link quality on the PC5 interface will be affectedsince many relays might be trying to access the same set of resources.Since the Uu quality is bad (that is the reason why the UE is lookingfor a Relay in the first place), this, combined with a bad PC5 linkquality, will bring down the user experience.

Moreover, another possibility to control relay activation would be touse dedicated signaling in said respect. In particular, when therelay-capable UE is interested in serving as a relay, it transmits acorresponding dedicated signaling message to the eNodeB, which then willhave the possibility to decide on whether the requesting relay UE shallbe activated as a relay or not. A corresponding response message canthen be sent back to the relay UE so as to give or deny permission toactivate the relay functionality. Although the exchange of dedicatedsignaling with the eNodeB has the advantage that the eNodeB canexplicitly control the number of relay UEs with activated relayfunctionality, this also entails some disadvantages. For instance, theuse of dedicated signaling is not possible for relay-capable UEs thatare in an idle state, since the dedicated signaling may only betransmitted to the eNB when being in a connected state i.e., having anactive connection with the eNodeB over which the dedicated signalingmessage is transmitted. Consequently, idle-state UEs will have totransition to the connected state, even if later on they will not bepermitted to activate their relay functionality, thereby wastingresources and battery. Furthermore, this approach leaves open whenexactly the relay capable UEs shall transmit the dedicated signaling tothe eNodeB so as to request serving as a relay. For example,transmitting dedicated signaling messages to the eNodeB might increasethe load at the eNodeB and might congest the Uu link unnecessarily,particularly if many relay-capable UEs are available in the radio celland repeatedly will seek the eNodeB for permission to activate the relayfunctionality. Additionally, an Idle-state UE transitioning toConnected-state to seek permission will need to stay in Connected-stateuntil the RRC connection is explicitly released by the eNB by, e.g.,sending a Connection Release message. Further, staying inConnected-state in the hope of serving as a relay for a remote UE maylead the relay UE to wait for a long time before a remote UE actuallyselects the relay UE to serve as relay. It would be better that therelay UE had an opportunity to act as Relay while being in Idle Modeitself.

Another possible solution for the relay activation would be combiningboth approaches mentioned above, namely to check additional relayrequirements and use dedicated signaling for getting the permission fromthe eNB. However, also in this combined approach there may be too many(unnecessary) requests transmitted to the eNB thereby congesting the Uulink as well as increasing the processing load in the eNB. Furthermore,also in this combined approach, idle-state UEs might establish the RRCconnection unnecessarily (i.e., transition to connected state) when theyare later denied by the eNodeB to activate their relay functionality incase no further relay(s) are actually needed. Moreover, this combinationapproach does not specify at which time(s) the dedicated signalingrequest should be transmitted to the eNodeB; repeatedly as long as therelay requirements are fulfilled.

The following exemplary embodiments are conceived by the inventors tomitigate one or more of the problems explained above.

Particular implementations of the various embodiments are to beimplemented in the wide specification as given by the 3GPP standards andexplained partly in the background section, with the particular keyfeatures being added as explained in the following pertaining to thevarious embodiments. It should be noted that the embodiments may beadvantageously used for example in a mobile communication system, suchas 3GPP LTE-A (Release 10/11/12/13) communication systems as describedin the Technical Background section above, but the embodiments are notlimited to its use in this particular exemplary communication networks.

The explanations should not be understood as limiting the scope of thedisclosure, but as a mere example of embodiments to better understandthe present disclosure. A skilled person should be aware that thegeneral principles of the present disclosure as laid out in the claimscan be applied to different scenarios and in ways that are notexplicitly described herein. For illustration purposes, severalassumptions are made which however shall not restrict the scope of thefollowing embodiments.

Furthermore, as mentioned above, the following embodiments may beimplemented in the 3GPP LTE-A (Rel.12/13) environment. The variousembodiments mainly provide a mechanism for a relay activation procedureperformed by relay UEs, such that other functionality (i.e.,functionality not changed by the various embodiments) may remain exactlythe same as explained in the background section or may be changedwithout any consequences to the various embodiments. This is true forinstance for the relay discovery procedure started after the relayfunctionality is activated, as well as for the exact procedures so as toestablish the direct sidelink connection over which they relay is takingplace, as well as for the exact procedure of how data is relayed betweenthe remote user equipment and the relay user equipment, etc.

A scenario may be assumed where user equipments are enabled to performProSe communication (ProSe-enabled UEs), i.e., direct D2D transmissionsdirectly between UEs without the detour via the eNodeB. Furthermore, atleast one of these (ProSe-enabled) UEs in the scenario shall supportrelay functionality as explained, e.g., in the background section forthe specific implementation in Release 13 of the 3GPP standard(s). Inother words, this relay UE (which is located in a radio cell andconnected to the corresponding radio base station controlling the radiocell) shall be capable of serving as a relay to other (ProSe-enabled)UEs (remote UEs) thereby allowing these remote UEs to connect, via therelay, to the eNB.

First Embodiment

In the following a first embodiment for solving the above problem(s)will be described in detail. Different implementations of the firstembodiment will be explained in detail below. According to the firstembodiment, the activation of the relay functionality in a relay-capableUE is improved.

A general implementation of the first aspect will be explained inconnection with the sequence diagram of FIG. 13 illustrating the relayUE behavior for the relay activation procedure to be performed in therelay UE so as to determine whether or not to activate the relayfunctionality.

The eNodeB is in control of when the relay-capable UEs in its radio cellshall start the respective relay activation procedure and thus can avoidunnecessary relay activations in its radio cell. In said respect, theeNodeB shall trigger the relay activation procedure(s) mainly when thereis a need in the radio cell for one or more additional relays. On theother hand, if there is no need in the radio cell for more relays, theeNodeB will not trigger the relay UEs in its radio cell to start therelay activation procedure. For said purpose, the eNodeB, whendetermining that further relays are necessary, will transmit a suitablebroadcast message in its radio cell preferably to be received by allin-coverage relay-capable UEs (e.g., also those UEs that are at theouter edges of the coverage area of the eNodeB to thereby substantiallyextend the coverage area by activating the relay functionality). Thisbroadcast message shall include or be interpreted by the receiving relayUEs as a trigger to start the relay activation procedure. For instance,the broadcast message may include an explicit relay trigger, e.g., aone-bit flag.

Furthermore, the eNB will select a suitable persistence check value(threshold), where the particular value is selected, e.g., depending onthe number of relays that are needed in the radio cell. By use of thepersistence check in the relay activation procedure, performed in therespective relay UEs (to be described later in detail), the eNodeB hascontrol on the number of relay UEs that might eventually activate therelay functionality. This persistence check value is also included inthe broadcast message broadcast in the radio cell by the eNodeB.Advantageously, instead of providing a separate indication in thebroadcast message that further relays are necessary (i.e., the separateindication triggering the start of the relay activation procedure in therelay UE), the presence of the persistence check value in the broadcastmessage may be already implicitly regarded as this indication thatfurther relays are necessary.

Correspondingly, from the perspective of one of these relay-capable UEsin the radio cell (i.e., see FIG. 13), the corresponding broadcastmessage with the relay activation trigger (to start performing the relayactivation procedure) and the persistence check value will be received.The reception of the broadcast message will trigger the relay activationprocedure in the relay UE, which comprises the following tests so as todetermine whether the relay functionality shall indeed be activated ornot.

The first check was already mentioned before, namely the persistencecheck performed by the relay UE based on the received persistence checkvalue in the broadcast message. The persistence check is to be performedby the relay UE, and only when it is successful, the activation of therelay functionality shall be possible. If not successful, the relay UEmay terminate the relay activation procedure and can continue to monitorfor another broadcast message that newly triggers the relay activationprocedure in the relay UE.

Another check to be performed during the relay activation procedure bythe relay UE is whether or not additional relay requirements, defined asa pre-requisite within the radio cell to act as a relay, are fulfilledby the relay UE. Again, only when the additional relay requirements arefulfilled, the activation of the relay functionality shall be possible.If the relay requirements are not fulfilled, the relay UE may terminatethe relay activation procedure and can monitor for another broadcastmessage that newly triggers the relay activation procedure in the relayUE.

As apparent from FIG. 13, when both checks are successfully finished bythe relay UE, the relay UE may activate its relay functionality and thusstart with the relay discovery procedure to announce its presence in thecell. The relay discovery procedure may be performed by the relay UE forinstance according to model A or model B, as explained in the backgroundsection. Correspondingly, the activated relay UE may be discovered andselected by a remote UE that needs to maintain or start a connection viathe relay to the eNodeB. Further details on subsequent procedures to beperformed after activating the relay functionality, such as thementioned relay discovery procedure, the relay select procedure and therelaying procedure as such, are omitted herewith; instead reference ismade to the background section as possible exemplary implementations ofthese procedures.

The relay activation according to the just explained first embodimentcomprises that the eNodeB specifically determines when relays are indeednecessary to then correspondingly transmit the broadcast message. Thisallows triggering the relay activation procedures in the relay UEs onlywhen it is indeed necessary. Consequently, the improved relay activationof the first embodiment thereby avoids that relay activation proceduresare started in the relay UEs unnecessarily, which saves processing onthe relay UE side. Furthermore, by implementing a persistence check inthe relay activation of the first embodiment, an additional level ofcontrol is provided for the eNodeB to limit the number of relay UEs(among all relay UEs in the radio cell that start the relay activationprocedure; i.e., preferably those that have not yet activated its relayfunctionality) that will activate the relay functionality.Correspondingly, only (more or less) the necessary number of additionalrelays will be activated, such that the relay discovery resources (thenused by the relay UEs after its activation) will not be congested withunnecessary relay discovery messages. This particular approach of relayactivation according to the first embodiment also refrains fromincluding an additional request for permission with the eNodeB therebyavoiding additional messages to be transmitted over the Uu link to theresponsible eNodeB. Correspondingly, the load at the eNodeB will not beincreased and congestion of the Uu link is avoided, while at the sametime allowing the eNodeB to have control on (at least) the number ofrelay UEs that will eventually activate the relay functionality.

In the following, further different and alternative implementations ofthe first embodiment will be explained.

As explained for the implementation of the first embodiment according toFIG. 13 (and which will also be part of the remaining implementationsaccording to the remaining figures), the relay UE will determine whetherit fulfills particular relay requirements defined in the radio cell.These particular relay requirements may be defined, e.g., by the eNodeB,or another responsible entity in the core network, such as MME or by theProSe function itself. In this case, corresponding information on theparticular relay requirements may be broadcast in the radio cell by theeNodeB, for instance in a suitable system information block (SIB).Alternatively, the requirements can be hardcoded in the UE, orpreconfigured by the operator, e.g., in USIM or configured by higherlayer signaling including NAS (non-access stratum) signaling or providedby the eNB in a dedicated signaling message to the relay UE when therelay UE was previously in the Connected-state.

The particular relay requirements may be different from radio cell toradio cell, and can be more or less stringent depending on the currentsituation. Some possible relay requirements will be presented in thefollowing, which can be employed separately or in combination. However,these parameters should not be regarded as being mandatory to be used asrelay requirements but shall be regarded as examples. For instance, thequality of the radio link between the relay UE and the eNodeB (i.e., theUu link) should not fall below a particular limit in order to ensureefficient relaying/forwarding by the relay UE. On the other hand, the Uulink quality could additionally be required to stay below apredetermined threshold when considering that relay UEs with a too goodUu quality might likely be far away from the cell edge or from coverageholes such that it might not be as interesting for the radio cell forsuch relay UEs to act as relays. Another possible relay requirement tobe fulfilled by prospective relay UEs might relate to the mobility of arelay UE. For instance, relay UEs moving at high speed are more likelyto get out of the reach of the remote UEs, thus forcing these remote UEsto select another relay UE.

Correspondingly, an upper limit for the mobility/speed of the relay UEmight be defined as a relay requirement to be fulfilled by any relay UEthat wants to act as a relay in the radio cell. A further possible relayrequirement refers to the battery level of the relay UE which preferablyshould be above a minimum threshold so as to ensure that the relay UE isable to perform as a relay for a sufficiently long time. For instance,if a relay UE with a limited remaining battery is selected as a relay,the packet forwarding for the remote UEs could quickly drain its batterypower. This is detrimental not only for the remote UEs, which have toselect a new relay, but also for the relay UE's normal operation whichis terminated earlier.

Other relay requirements may for instance refer to an overload situationin the relay UE, which will prevent or seriously inhibit the relay UE toserve as a relay for a remote UE. The functional definition of“preventing or seriously inhibiting the relay to serve as a relay” shallnot be interpreted narrowly in that it is impossible for a relay userequipment to serve as a relay for a further single remote userequipment. Rather, an overload situation may be defined flexibly for arelay user equipment, for instance by defining particular limits atwhich it is understood that an efficient operation of the relay userequipment is ensured. The overload might refer to any of the hardware orsoftware components of the relay user equipment, such as the processor,memory, ports, logical channel IDs, available bandwidths inuplink/downlink, etc. The overload is characterized as being temporarysince it may rapidly change.

Correspondingly, a set of one or more of these or other relayrequirements might need to be checked by any relay UE during the relayactivation procedure of the implementation of the first embodiment.

For the specific implementation of FIG. 13, it was assumed that the twochecks are performed basically in parallel. However, FIG. 14 illustratesa different implementation of the first embodiment, where these twodeterminations are performed subsequently, the particular order beingirrelevant for the functioning of the present disclosure. In theexemplary implementation of FIG. 14 the simple persistence check isperformed first, and only in case this persistence check is successful,the relay UE then continues to check whether the relay requirements arefulfilled or not. Consequently, if already the persistence check fails,the relay activation procedure may be stopped immediately since there isno need to further check whether the relay requirements are fulfilled,which saves processing at the relay UE.

Similarly, although not illustrated in the figures, the relay UE mayfirst check whether the relay requirements are fulfilled, and, only incase the relay requirements are fulfilled, may continue to perform thepersistence check. Thus, if already the relay requirements check fails,the relay activation procedure may be terminated since there is no needto further perform the persistence check, which saves processing at therelay UE.

Moreover, as was described above, the relay UE performs a persistencecheck during the relay activation procedure based on the persistencecheck value provided by the eNodeB in the broadcast message. Thepersistence check value, which can be seen as a threshold to which asubsequently generated random value will be compared to pass or not passthe persistence check, is determined in the eNodeB, e.g., based on thenumber of relay UEs which the eNodeB wants to have activated. In oneimplementation, the eNodeB can decide on the number of relay UEs thatare required in the cell based on many factors including a feedback fromthe ProSe function which the eNB can receive through proprietaryinterface(s) or through core network elements like MME, or purely basedon OAM (Network Operations and Management) configuration from thenetwork, e.g., stating that certain number of relays are required everysquare kilometer, or purely based on its own deduction from the numberof UEs running public safety services in the cell (which in turn isclear from the CQI classes of the bearers being served by the eNB)and/or some statistical calculation on how many relay UEs are generallyrequired per certain number of UEs running public safety services; or,the eNodeB determination on the number of relay UEs that are required inthe cell could purely be based on the UEs reporting their requirement(s)for relay service. Another example could be that the UE reporting theirrequirement for relay service could be based on the in-coverage publicsafety UEs experiencing bad radio quality (on Uu interface) and theeNodeB extrapolating this figure to include a possible number of out ofcoverage UEs.

Persistence checks are already used in the prior art, 3GPP, standards,although for other purposes. For instance, technical standard TS 25.321,current version v12.2.0, defines a persistence value Pi, which is usedso as to control the instant of time when a UE is allowed to access theRACH channel when, e.g., a previous transmission was not deemedsuccessful. By spreading the access in the time domain, the number ofUEs that access the RNC at any given time is tightly controlled by theRNC by adjusting the value of persistence value Pi.

According to variants of the first embodiment, the persistence check maybe performed in a similar manner. Thus, a range of values (e.g., between0 and 1) is defined within which the persistence check value is selectedby the eNodeB. Correspondingly, the relay UE(s) will generate during therelay activation procedure a random value within this same range ofvalues. In order to pass the persistence check, the randomly generatedvalue of the relay UE will be compared to the persistence check valueselected by the eNodeB. One possibility is to define that thepersistence check is successful, in case the randomly generated value issmaller than or equal to the persistence check value provided by theeNodeB; or vice versa in case the randomly generated value is largerthan the persistence check value provided by the eNodeB. For instance,by selecting a suitable persistence check threshold, the eNodeB cancontrol the percentage of persistence checks that will be successful ornot. When assuming that the persistence check is successful when therandomly generated value is smaller than or equal to the persistencecheck value, the eNodeB may limit the successful persistence checks to alow percentage by setting the persistence check threshold to a lowvalue, e.g., 0.1; correspondingly, setting the persistence checkthreshold to a middle value, such as 0.5, will allow the eNodeB tocontrol that only about half of the persistence checks are successful.Correspondingly, the persistence check provides a simple and effectivemechanism to leave the eNodeB some control on the number of relay UEsthat will/can activate its relay functionality, without having to forcethe relay UEs to directly seek permission from the eNodeB via dedicatedsignaling.

Nevertheless, although the previous implementations of the firstembodiment do not have to have a dedicated request for permission fromthe eNodeB, an alternative implementation of the first embodimentincludes such an additional level of control. In particular, thesequence diagram of FIG. 15 illustrates such an exemplaryimplementation, based on the previously discussed implementation of FIG.14. In addition to the persistence check and relay requirement check,the relay activation procedure, according to this alternativeimplementation of the first embodiment illustrated in FIG. 15,additionally comprises that the relay UE requests permission from theeNodeB as to whether it is allowed to activate its relay functionalityor not. This additional request for permission can be for exampleperformed after successfully concluding both checks of the relayactivation procedure, as assumed for the implementation illustrated inFIG. 15. Consequently, the eNodeB will then have the opportunity tospecifically deny or a grant the permission for each of the requestingrelay user equipments one by one. For instance, this would beadvantageous in scenarios where the eNodeB does not exactly know howmany relay-capable UEs are in its radio cell but where the eNodeB stillwants to ensure that the number of UEs with an activated relayfunctionality stays below a particular limit.

Correspondingly, after transmitting a corresponding message (e.g.,termed relay activation request message) to the radio base station, therelay UE will await and eventually receive a corresponding responsemessage (e.g., termed relay activation response message) which includesthe response from the radio base station, i.e., whether or not the relayUE is allowed to activate its relay functionality. Following the contentof this response message, the relay UE will thus activate or notactivate its relay functionality. In one implementation, both therequest and response message can either be designed as an RRC message(e.g., SidelinkUEInformation of 3GPP TS 36.331, for details see later)or as a MAC CE (Control Element) with a specified LCID (Logical ChannelIdentify) each for the request and response message.

This relay activation request message may not only carry the request forpermission to activate the relay functionality of the relay UE (and therequest for radio resources, see implementations of the first embodimentdescribed later), but may also comprise further information as will beexplained in the following.

For instance, the message may indicate that the purpose of seekingpermission is to act as a relay.

Furthermore, this relay activation request message transmitted from therelay UE to the eNodeB may comprise information on the one or moreservices that may be provided by the relay UE to remote user equipments.For instance, the services may be public safety services or non-publicsafety services. In any case, by providing such information on theoffered services, the eNodeB may determine a priority associated withthe respective one or more offered services and may thus base itsdecision of whether to permit or not permit the relay UE to activate itsrelay functionality based on such information. As an example, thepotential relay UE may indicate its intention to serve Medical EmergencyPersonnel for Voice-specific call in a flat layout, and the eNB alreadythat received and approved 5 such relays already knows that more relayscannot be accommodated (without creating interference) in the cell andthat the 5 relays are already sufficient based on its knowledge, e.g.,from Proximity services, such that the eNodeB rejects the new request(s)to become a relay.

Similarly, the relay UE may include group identification information onthe one or more services that may be provided by the relay UE to remoteuser equipments. This group identification information allowsidentifying the group to which each of the one or more offered servicesbelongs to.

According to a particular implementation in the 3GPP standardenvironment of Release 12, 13, the SidelinkUEInformation message(already defined in the technical standard TS 36.331, current version12.6.0, clause 6.2.2 incorporated herein by reference) can be reused insaid respect. Correspondingly, this SidelinkUEInformation message can beextended with additional information element(s) to be able to indicatesome additional information as explained above, e.g.:

-   -   the purpose of seeking permission is to act as a relay    -   that the request for resources refers to resources for the relay        discovered procedure and not for, e.g., commercial discovery        procedure or even the Release-12 direct (D2D) communication        between two ProSe UEs; thus, a corresponding additional        information element would allow to simultaneously request        resources for the relay discovery procedure as well as for the        Release 12 direct discovery procedure and/or Release 12 D2D        communication.

A corresponding example of an extended definition of theSidelinkUEInformation message is given below.

SidelinkUEInformation message

-- ASN1START SidelinkUEInformation-r12 ::= SEQUENCE { criticalExtensionsCHOICE { c1 CHOICE { sidelinkUEInformation-r12 SidelinkUEInformation-r12-IEs, spare3 NULL, spare2 NULL, spare1 NULL },criticalExtensionsFuture SEQUENCE { } } } RequestPurpose-r13-IEs::=BOOLEAN, SidelinkUEInformation-r12-IEs ::= SEQUENCE {commRxInterestedFreq-r12 ARFCN-ValueEUTRA-r9 OPTIONAL,commTxResourceReq-r12 SL-CommTxResourceReq-r12 OPTIONAL,discRxInterest-r12 ENUMERATED {true} OPTIONAL, discTxResourceReq-r12INTEGER (1..63) OPTIONAL, lateNonCriticalExtension OCTET STRINGOPTIONAL, nonCriticalExtension SEQUENCE { } OPTIONAL }SL-CommTxResourceReq-r12 ::= SEQUENCE { carrierFreq-r12ARFCN-ValueEUTRA-r9 OPTIONAL, destinationInfoList-r12SL-DestinationInfoList-r12 } SL-DestinationInfoList-r12 ::= SEQUENCE(SIZE (1..maxSL-Dest-r12)) OF SL- DestinationIdentity-r12SL-DestinationIdentity-r12 ::= BIT STRING (SIZE (24)) -- ASN1STOP

SidelinkUEInformation field descriptions commRxInterestedFreq Indicatesthe frequency on which the UE is interested to receive sidelink directcommunication. commTxResourceReq Indicates the frequency on which the UEis interested to transmit sidelink direct communication as well as thesidelink direct communication transmission destination(s) for which theUE requests E-UTRAN to assign dedicated resources. destinationInfoListIndicates the destination which is identified by the ProSe Layer-2 GroupID as specified in TS 23.303 [68]. discRxInterest Indicates that the UEis interested to monitor sidelink direct discovery announcements.discTxResourceReq Indicates the number of resources the UE requiresevery discovery period for transmitting sidelink direct discoveryannouncement. It concerns the number of separate discovery message(s)the UE wants to transmit every discovery period. RequestPurpose Couldhave two values: Add (true) or New (false). “Add” indicates if the UEneeds to support (receive and/or transmit) SL for both Rel. 12 and Rel.13 (Relay Discovery) purpose simultaneously and therefore, the newrequest is on top of the resources that it already has been configuredwith and which it intends to continue to use. “New” indicates that therequest is only to grant resources requested in this message.

According to further implementations of the first embodiment, the relayactivation procedure will also take into account whether radio resourcesare already available for performing the relay discovery procedure(which is started subsequent to activating the relay functionality). Insaid connection, it should be noted that in the standardization no finalagreement has yet been achieved on when and how the radio resources tobe used for relay discovery are defined and provided to the relay UE(s).One possible implementation is to broadcast a suitable relay discoveryresource pool, providing radio resources that are to be used inconnection with the relay discovery procedure to be performed by therelay UE. In turn, specific radio resources may then be eitherautonomously selected by the relay UE from such a suitable relaydiscovery resource pool, or the specific radio resources (from withinthis relay discovery resource pool) have to be scheduled by the eNodeB(upon being requested by the relay UE). The relay UEs in the radio cellmay be configured to either be allowed to autonomously select radioresources from such a pool or may need to request dedicated radioresources from the eNodeB first. In any case, the relay UE willdetermine whether or not radio resources are already configured andavailable to be used by the relay UE for the relay discovery procedureto be performed upon activating its relay functionality. Then, in casesuitable radio resources are indeed available to the relay UE, the relayUE may continue with the relay activation procedure (e.g., activate therelay functionality). On the other hand, in case no suitable radioresources are available to the relay UE for the relay discoveryprocedure, the relay UE may request such radio resources from the radiobase station, and the radio base station will correspondingly receiveand respond by assigning suitable radio resources for the relaydiscovery. Upon being assigned suitable radio resources, the relay UEmay continue with the relay activation procedure (e.g., activate therelay functionality).

As mentioned above, one possible implementation of how to request radioresources is the use of the SidelinkUEInformation message. The relay UEmay signal within this message that resources are requested for therelay discovery procedure. Furthermore, the SidelinkUEInformationmessage shall be extended by an information element to request resourcesfor relay discovery procedure such that within the sameSidelinkUEInformation message the relay UE may additionally requestresources for commercial discovery and/or for Rel.12 D2D communication.A possible exemplary implementation of the SidelinkUEInformation messageis provided below.

SidelinkUEInformation message

-- ASN1START SidelinkUEInformation-r12 ::= SEQUENCE { criticalExtensionsCHOICE { c1 CHOICE { sidelinkUEInformation-r12 SidelinkUEInformation-r12-IEs, spare3 NULL, spare2 NULL, spare1 NULL },criticalExtensionsFuture SEQUENCE { } } }SidelinkUEInformationList-r13-IEs::= SEQUENCE (SIZE (1..maxSL-purpose))OF SL-DestinationIdentity-r12, SidelinkUEInformation-r12-IEs ::=SEQUENCE { commRxInterestedFreq-r12 ARFCN-ValueEUTRA-r9 OPTIONAL,commTxResourceReq-r12 SL-CommTxResourceReq-r12 OPTIONAL,discRxInterest-r12 ENUMERATED {true} OPTIONAL, discTxResourceReq-r12INTEGER (1..63) OPTIONAL, lateNonCriticalExtension OCTET STRINGOPTIONAL, nonCriticalExtension SEQUENCE { } OPTIONAL }SL-CommTxResourceReq-r12 ::= SEQUENCE { carrierFreq-r12ARFCN-ValueEUTRA-r9 OPTIONAL, destinationInfoList-r12SL-DestinationInfoList-r12 } SL-DestinationInfoList-r12 ::= SEQUENCE(SIZE (1..maxSL-Dest-r12)) OF SL- DestinationIdentity-r12SL-DestinationIdentity-r12 ::= BIT STRING (SIZE (24)) -- ASN1STOP

SidelinkUEInformation field descriptions commRxInterestedFreq Indicatesthe frequency on which the UE is interested to receive sidelink directcommunication. commTxResourceReq Indicates indicates the frequency onwhich the UE is interested to transmit sidelink direct communication aswell as the sidelink direct communication transmission destination(s)for which the UE requests E-UTRAN to assign dedicated resources.destinationInfoList Indicates the destination which is identified by theProSe Layer-2 Group ID as specified in TS 23.303 [68]. discRxInterestIndicates that the UE is interested to monitor sidelink direct discoveryannouncements. discTxResourceReq Indicates the number of resources theUE requires every discovery period for transmitting sidelink directdiscovery announcement. It concerns the number of separate discoverymessage(s) the UE wants to transmit every discovery period.SidelinkUEInformationList Indicates if the UE needs to support (receiveand/or transmit) SL for both Rel. 12 and Rel. 13 (Relay Discovery)purpose simultaneously.

One particular exemplary implementation of how to check and request forradio resources is illustrated in FIG. 16, which is based on theprevious implementation discussed in connection with FIG. 15 thatadditionally requires the relay UE to first seek permission from theeNodeB before activating the relay functionality. However, it should benoted that alternatively this particular implementation will also bepossible without the additional request for permission performed by therelay UE; i.e., by extending the relay activation procedure as explainedin connection with FIG. 13 and FIG. 14 with the above mentionedadditional steps where the relay UE determines whether radio resourcesare needed and, if so, requests and receives the grant of radioresources from the eNodeB.

A corresponding extension of the exemplary implementation explained inconnection with FIG. 14 is illustrated by the sequence diagram of FIG.17, albeit with a slight variation. As explained already in connectionwith FIG. 16, the relay UE additionally determines whether radioresources are already available for the relay discovery procedure, andin case no such resources are available, the relay UE continues with acorresponding request towards the eNodeB. Then, depending on whetherradio resources are assigned or not, the relay UE continues to activatethe relay functionality (in case radio resources are indeed assigned) orterminates the relay activation procedure (in case no radio resourcesare assigned). Therefore, in this advantageous implementation, therequest for resources can be reused for seeking permission to activatethe relay functionality or not. In particular, the request for radioresources transmitted by the relay UE to the radio base station can beseen as an implicit request for permission, since the eNodeB will havethe opportunity to give or deny permission to activate the relayfunctionality to particular relay UEs by assigning or not assigningradio resources in response to the radio resource request.Correspondingly, when the eNodeB decides that the requesting relay UEshall not activate its relay functionality, it may simply assign noresources to the relay UE (either by not transmitting a response messageback to the relay UE, or by responding with a corresponding informationthat no radio resources are assigned) which thus is interpreted by therelay UE in that the eNodeB does not give permission to activate therelay functionality. On the other hand, when the eNodeB decides that therequesting relay UE shall indeed activate its relay functionality, byproviding corresponding information on the assigned radio resources, theeNodeB will implicitly permit the relay UE to activate the relayfunctionality.

In the implementation according to FIG. 17 the additional steps fordetermining and requesting radio resources are provided after the twochecks (relay requirements check and persistence check), since thissequence avoids transmitting additional messages via the Uu link to theradio base station for the case that one of said two checks fails.Nevertheless, theoretically, these steps for determining and requestingradio resources may alternatively be provided before the persistencecheck and/or the relay requirement check such that the persistence checkand/or the relay requirement check is/are only performed after radioresources are available (either, by having been available before, orafter having requested them).

A further variant of the first embodiment will be explained inconnection with FIG. 18, which additionally takes into account whether arelay UE is in an RRC idle or RRC connected state. In order to explainthis additional improvement, it is exemplarily assumed that the relayactivation procedure also includes the steps where the relay UEdetermines whether the resources are available for the relay discoveryprocedure or not as explained before. However, it should be noted thatthis additional improvement of taking into account the idle/connectedstate of the UE can also be included standalone in the relay activationprocedure (i.e., without having to have the radio resourcedetermination). In general, the relay UE may be in RRC connected or idlestate when performing the relay discovery procedure while it will benecessary for the relay UE to transition to the connected state once itis selected by a remote UE to act as a relay since the relay UE needs toforward and receive data on the Uu link for the relaying. Thus, forinstance the relay UE may perform the relay discovery procedure in RRCidle state, however will then transition to the RRC connected state onceit has been selected to become a relay for a remote UE. Nevertheless, incase of relaying eMBMS traffic to the remote UE, the relaying may bedone by the relay UE in RRC idle as well. Moreover, for thoseadvantageous implementations of the first embodiment involving a directdedicated signaling with the radio base station (e.g., for requestingresources and/or requesting permission to activate the relayfunctionality), the relay UE shall be in a RRC connected state so as tobe able to perform this dedicated signaling. Therefore, in advantageousimplementations of the first embodiment a relay UE in idle state willfirst transition to the connected state before continuing/finalizing therelay activation procedure. To said end, the relay UE may firstdetermine the particular RRC state, idle or connected, and in case therelay UE is in the idle state, the relay UE would have to perform thecorresponding procedure to transition to the connected state, beforecontinuing with the relay activation procedure (in the particularexample of FIG. 18 so as to request radio resources from the radio basestation by dedicated signaling).

It should be noted, that the step of transitioning from the idle stateto the connected state can be performed, e.g., by a RRC connectionprocedure. A particular implementation of such a procedure fortransitioning from RRC idle to RRC connected state is alreadystandardized in 3GPP, e.g., the RRC connection establishment proceduredefined by the technical standard TS 36.331, current version v12.6.0, inclause 5.3.3, incorporated herein by reference. In summary, afterperforming a random-access by the relay UE, the relay UE will transmitan RRC connection request message (RRCConnectionRequest message) to theeNodeB which in turn then responds by transmitting an RRC connectionresponse message (RRCConnectionSetup), including the necessaryparameters, so as to establish the RRC connection between the relay UEand the eNodeB; this further includes a UE context based on a UEspecific RRC level identity called C-RNTI. The UE is further identifiedon the Uu link based on this C-RNTI which is retained both in the UE andthe eNodeB until the RRC Connection is released. It should be noted thatonce a relay UE has transitioned to the connected state, it will usuallystay in the connected state until the connection is released by theeNodeB or until the UE has to transition to Idle Mode after a Radio Linkfailure wherein a Re-establishment of RRC Connection was not possible(clause 5.3.7 of TS 36.331).

Consequently, relay UEs in idle state may also be enabled tosuccessfully finish the relay activation procedure particularly forthose implementations of the first embodiment where dedicated signalingis required so as to successfully finish the relay activation procedure.

According to further variants of the first embodiment, the ProSefunction in the network will also have control on the relay activationas will be explained in the following in connection with the exemplarilyillustration of FIG. 19. In particular, the ProSe function will transmita corresponding relay initiation message to the relay UE as a firsttrigger such that the relay UE will start monitoring corresponding radioresources via which the broadcast message will be transmitted by theeNodeB. In particular, in order to maintain some network control overthe relay situation in the respective radio cells, the ProSe function(for example additionally in consultation with the ProSe applicationserver) may decide that particular radio cells shall provide relays(even though further control might rest with the eNodeB as explainedbefore), e.g., when it is informed of some special public safetyscenario in certain geographical areas/cells. By transmitting acorresponding relay initiation message to the relay-capable UE(s) in theradio cell, the ProSe function will thus trigger the relay UE tobroadcast messages from the eNodeB (e.g., as defined in clause 5.2 of TS36.331, incorporated herein by reference) to actually trigger the relayactivation procedure in the relay UE. As apparent from FIG. 19, therelay UE will thus monitor and eventually receive the relay initiationmessage in which case it will start monitoring radio resources so as toreceive the broadcast message from the eNodeB.

Alternatively, or in addition, the ProSe function might transmit asimilar message to the eNodeB to initiate relay activation through theeNodeB. The eNodeB in turn might then immediately conclude from thecorresponding relay initiation message received from the ProSe functionthat further relays are necessary, and will thus transmit the previouslydiscussed broadcast message in the radio cell to the relay UE(s).Alternatively, the eNodeB, upon receiving such relay initiation messagefrom the ProSe function, will then determine whether indeed furtherrelays are necessary, e.g., based on the number of remote UEs in theradio cell that have a bad radio link with the eNodeB, and/or based onthe number of remote UEs running public safety services and the radiocell. To explain further, an eNodeB may decide on the number of relayUEs that are required in the cell based on possibly many factorsincluding a feedback from the ProSe function which the eNB can receivethrough proprietary interface(s) or through core network elements likeMME, or purely based on OAM configuration from the network, e.g.,stating that certain number of relays are required every squarekilometer, or purely based on its own deduction from the number of UErunning PS services in the cell (which in turn is clear from the CQIclasses of the bearers being served by the eNB) and some statisticalcalculation on how many relay UEs are generally required per certainnumber of UEs running PS services; or, the eNodeB determination on thenumber of relay UEs that are required in the cell could purely be basedon UE reporting their requirement for Relay service. The last, i.e., theUE reporting their requirement for Relay service could be based on thein-coverage public safety UEs experiencing bad radio quality (on Uuinterface) and the eNodeB extrapolating this figure to include apossible number of out of coverage UEs. This determination may beperformed periodically by the eNodeB after receiving the relayinitiation message.

This determination by the eNodeB as to whether further relays arenecessary, as just explained above, can likewise be performed inimplementations of the first embodiment that do not comprise the relayinitiation message exchange from the ProSe function, e.g., in theimplementations explained in connection with FIGS. 13 to 18 (andcorresponding variants thereof). In that case, the eNodeB will alsoperiodically determine whether further relays are necessary or not.

A further advantageous and extended version of the implementation of thefirst embodiment as explained in connection with FIG. 19, will beexplained in connection with FIG. 20. As apparent therefrom, anadditional determination has been introduced into the relay activationprocedure namely as to whether a relay stop message is received from theProSe function. In an opposite manner as for the relay initiationmessage, the ProSe function (for example in addition in consultationwith the ProSe Application Server) may decide that particular radiocells shall not provide any relays anymore and may thus correspondinglytransmit a relay stop message in the radio cell. Correspondingly, incase the relay UE receives such relay stop message, it will stopmonitoring for the broadcast message from the eNodeB.

According to further advantageous implementation of the firstembodiment, the broadcast message is extended with additionalinformation that is useful for performing the relay activation procedurein the relay UE. As has been explained before, the broadcast messageshall include the persistence check value as well as function as thetrigger for the relay UE to start the relay activation procedure.Additionally, the broadcast message may include the additional relayrequirements to be fulfilled in the radio cell and which are checked bythe relay user equipment during the relay activation procedure. Asmentioned before in a particular implementation of the first embodiment,the eNodeB may be the entity to determine the particular relayrequirements for its cell, and will thus be able to correspondinglyinclude information on the relay requirements in the broadcast messagetransmitted in its radio cell. Advantageously, the presence of thisinformation on the relay requirements in the broadcast message can beimplicitly regarded as the indication that further relays are necessaryin the radio cell (i.e., can be implicitly regarded as the trigger tostart the relay activation procedure in the relay user equipment) suchthat a separate indication in said respect is not necessary.

Alternatively or in addition, according to a further implementation ofthe first embodiment, the broadcast message may be extended withinformation on the radio resources that can be used by the relay UE toperform the relay discovery procedure after activating its relayfunctionality. For instance, the information on the radio resourcesprovided by the eNodeB in the broadcast message can be the same orsimilar to the previously discussed relay discovery resource pool fromwhich the user equipment then can, e.g., autonomously select radioresources to perform the relay discovery. On the other hand, whenassuming that the relay discovery resource pool information was alreadyprovided to the relay UE (e.g., via the system information), thisinformation on the radio resources provided by the eNodeB in thebroadcast message can refer to only part of the whole relay discoveryresource pool. In any case, by providing corresponding radio resourcesalready in the broadcast message transmitted by the eNodeB, it will notbe necessary to additionally request radio resources later on during therelay activation procedure as for instance explained in connection withsome of the implementations of the first embodiment where the relayactivation procedure comprises steps to determine whether radioresources are already available for the relay discovery procedure (seee.g., FIGS. 16 and 17). Advantageously, the presence of such resourceinformation in the broadcast message can be implicitly regarded as theindication that further relays are necessary in the radio cell (i.e.,can be implicitly regarded as the trigger to start the relay activationprocedure in the relay user equipment) such that a separate indicationin said respect is not necessary.

A further advantageous implementation of the first embodiment providesthe additional mechanism of deactivating relay functionality asnecessary, such that a relay UE will not unnecessarily keep sendingrelay discovery messages thereby further depleting its battery andcongesting the corresponding relay discovery radio resources.Correspondingly, for this implementation it is assumed that relay UEshave already activated its relay functionality and may or may not serveas a relay for other remote UEs. According to this implementation, theeNodeB may decide to no longer provide any relays in its radio cell orsimply may decide to reduce the number of relays in its radio cell. Inany case, a mechanism is provided by this implementation so as to allowthe eNodeB to deactivate all or only specific relay(s) UE. In saidrespect, the eNodeB may use a corresponding relay deactivation commandmessage which may be either broadcast in the radio cell or be directlytransmitted to the relevant relay UE(s) which relay functionality shallbe deactivated. In more detail, in case the eNodeB would like todeactivate all relays in its radio cell, it may decide to broadcast acorresponding deactivation command to be received by all relay UEs inits radio cell, where each of the relay UEs will follow the command and,when having relay functionality activated, will deactivate the relayfunctionality. On the other hand, the eNodeB may also use dedicatedsignaling respectively with only one relay, so as to deactivate therelay functionality of said relay UE. However, it should be noted thatthe eNodeB will not be able to reach relay UEs that are in an idle stateby using the dedicated signaling. In this case, the eNodeB mayalternatively or additionally use broadcast signaling to deactivate therelay functionality specifically of only those relay UEs with activatedrelay functionality that are in an idle state. Relay UEs in idle statewill receive the broadcast signaling with the special indication andwill in response deactivate the relay functionality (if it wasactivated). As a further example, the broadcast message can even use thepersistence check mechanism, as already described, to deactivate onlypart of the whole set of activated relays.

Alternatively, the relay deactivation may also be initiated by the relayUE, e.g., when the relay UE has stopped serving as a relay for anyremote UE for a particular period of time or in case the ProSe functioninforms the UE to stop being a relay. In this case, the relay UE maytransmit a corresponding relay deactivation request message to theeNodeB, which in turn may then decide whether indeed the relay UE shalldeactivate its relay functionality or not. Correspondingly, the eNodeBwill transmit a response message back to the relay UE providing thecorresponding deactivation command or not.

In particular implementations according to the 3GPP environment, theRRCConnectionReconfiguration message can be reused to serve as the relaydeactivation command.

Second Embodiment

In the following a second embodiment is presented which deals with thefollowing problem. In particular, the current standardization for theProSe relay functionality does not specify when a remote user equipmentbegins and stops sending data via the PC5 interface. In other words, noparticular agreements were reached in the current 3GPP standardizationas to when a remote UE shall start transmitting/receiving data via therelay connection instead of via the direct Uu link with the eNodeB. Itis also unclear when the remote UE should switch back to the Uu link.

It should be noted that in general, the remote UE shall prefer using theUu link, instead of the PC5 link. Consequently, the relay operationshould be started only when the Uu link is quite week/unsustainable orinefficient and/or should be stopped when the remote UE can be servedagain by the eNodeB directly over the Uu link. This is since a dynamicscheduling based on various measurement reporting, CSI reporting, etc.,is possible and performed on the Uu interface but not for the PC5 link.

For discussing the second embodiment, it is assumed that the remote UEwill eventually select a relay UE from among one or more possiblediscovered relay UEs (discovered by performing relay discovery as, e.g.,explained in the background section). Correspondingly, this will alsoinclude establishing a corresponding direct connection with the relay UEover which the communication can then be relayed. In particular, such adirect connection can be established as explained in the backgroundsection, e.g., by establishing the layer-2 link between the relay UE andthe remote UE.

It is now important to decide at which point of time (after the relay UEhas been selected by the remote UE and the corresponding directconnection between the relay UE and the remote UE has been established)the actual data switch (to the PC5 interface) should take place. Thereare several options. For instance, a remote UE may autonomously decideif at all and when to start transmitting/receiving data over the PC5link. This autonomous decision can be based on a number of differentoptions including Uu and/or PC5 link quality, transmission powerrequired on each link and other similar considerations.

For example, a remote UE may start transmitting data over the PC5 linkif the corresponding Uu link quality between the remote UE and the radiobase station falls below a specific configured threshold. As a stillfurther alternative, a remote UE may be configured to immediately starttransmitting data over the PC5 link after having successfullyestablished the layer-2 link with the relay UE.

In any case, the remote UE may inform the eNodeB about the path switchto the PC5, such that the eNodeB in turn may be able to release anddeconfigure existing data bearers such that the communication of theremote UE now continues to be relayed to the remote UE via the relay UE.

Alternatively, the remote UE may directly start using the communicationlink with the selected relay UE without having informed the eNodeBbeforehand, for instance by sending Unacknowledged data packets (e.g.,PDCP SDUs) to the relay UE. In turn, the relay UE will then inform theeNodeB, which in turn will start sending the downlink (PDCP)unacknowledged data packets to the remote UE via the selected relay userequipment. The knowledge of “which” remote UE may be conveyed to eNB viathe relay, using the C-RNTI assigned to the UE on the Uu link that wasbeing used immediately before the UE moved to PC5.

In general, all the data switches (from Uu to PC5, and from PC5 to Uu)that are performed by the remote UE (and the relay UE) should beinformed to the application layer from the access stratum. This is sincethe application layer in this case, e.g., Proximity Function may need tomap the radio network layout (e.g., Cell and tracking area Id) to itsown infrastructure.

Further solutions are provided to successfully move the UE's connectionfrom the PC5 link back to the Uu link. In this regard, it is now assumedthat the relay UE is acting as the relay for the remote UE such that thecommunication of the remote UE is relayed between the eNodeB and theremote UE via the relay UE. According to one solution, a handover-likeprocedure could be used to move the remote user back to the Uu link. Inparticular, the remote UE may send the usual measurement reports to theeNodeB via the relay connection. For instance, the old measurementconfiguration, received previously when being connected to the eNodeBvia the Uu link, can be maintained after the PC5 data switch and thusmay be used for measuring the Uu link even when the remote userequipment is on the PC 5 link. A corresponding handover message (such asthe RRCConnectionReconfiguration message with the MobilityControlInfo ofTS 36.331, current version 12.6.0, incorporated herein by reference)could be sent by the eNodeB via the relay UE to the remote UE. In thisparticular case, the Uu link to which it should be switched back couldbelong to the same old (source) cell or could belong to any otherneighbor cell. However, the solution has a problem in that the UE andthe eNodeB retain the Uu context(s) (including the configuration) aswell as that the RRC message signaling via the PC5 link isdisadvantageous since the link is supposed to be used only to convey thehigher layer data (e.g., the application data), and the lower layersignaling transport on PC5 may be avoided since in this regard the samecomplexity may need to be supported as to maintain the RRC Connection onthe Uu interface.

On the other hand, as mentioned before, the remote UE may switch back tousing the Uu link (instead of the PC5 interface link) when the Uu linkquality is sufficiently good. In said case, the RRC connectionestablishment procedure can be performed by the remote UE,advantageously indicating as the cause of the connection establishmentthat the remote UE would like to move from the PC5 interface to the Uuinterface. According to an exemplary implementation in the 3GPPstandards environment, the RRC connection establishment procedureaccording to the technical standard TS 36.331 (version v12.6.0, inclause 5.3.3, incorporated herein by reference) can be reused.

In order to properly determine by the remote UE when the Uu link isbetter again, the remote UE may for instance perform certain radio linkmeasurements such as those involving RSRP and/or RSRQ, and/or pathlossinformation, etc. A corresponding minimum threshold can be defined foreach of the respective radio link measurements, which the Uu link mustfulfill so as to be determined as being sufficiently good to switch backto. For example, each of the predefined threshold could be configured bythe eNodeB, and corresponding information on the thresholds could beprovided to the remote UE while it was still reachable via the Uu link(i.e., prior to performing the data switch to the PC5 link).

According to an alternative implementation of the second embodiment, theremote UE will use the cell selection criteria (as defined in TS 36.304)to determine when the Uu is good enough to initiate a data switch backto said Uu link. In particular, the 3GPP technical standard TS 36.304,current version 12.5.0, defined in clause 5.2.3.2 cell selectioncriteria, which are used to evaluate a cell for the cell selectionprocedure described in clause 5.2, incorporated herein by reference. Thefollowing is an excerpt of clause 5.2.3.2 of said standard TS 36.304:

The cell selection criterion S is fulfilled when:

Srxlev>0AND Squat>0

where:

Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P_(compensation) −Qoffset_(temp)

Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))−Qoffset_(temp)

where:

Srxlev Cell selection RX level value (dB) Squal Cell selection qualityvalue (dB) Qoffset_(temp) Offset temporarily applied to a cell specifiedin (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP) Q_(qualmeas)Measured cell quality value (RSRQ) Q_(rxlevmin) Minimum required RXlevel in the cell (dBm) Q_(qualmin) Minimum required quality level inthe cell (dB) Q_(rxlevminoffset) Offset to the signalled Q_(rxlevmin)taken into account in the Srxlev evaluation as a result of a periodicsearch for a higher priority PLMN while camped normally in a VPLMNQ_(qualminoffset) Offset to the signalled Q_(qualmin) taken into accountin the Squal evaluation as a result of a periodic search for a higherpriority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX)-P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power level anUE may use when transmitting on the uplink in the cell (dBm) defined asP_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of the UE(dBm) according to the UE power class as defined in [TS 36.101]

The signaled values Q_(rxlevminoffset) and Q_(qualminoffset) are onlyapplied when a cell is evaluated for cell selection as a result of aperiodic search for a higher priority PLMN while camped normally in aVPLMN. During this periodic search for higher priority PLMN the UE maycheck the S criteria of a cell using parameter values stored from adifferent cell of this higher priority PLMN.

Correspondingly, the remote UE may reuse this cell selection criterionso as to determine when the Uu link is sufficiently good again so as toswitch back to same.

According to a further improvement, a new monitoring behavior isprovided for the remote UE. In particular, since the remote UE issupposed to only be running public safety applications, it may onlyselectively need to monitor those specific discovery resource pools thatare specifically defined for being used for relay discovery for thesepublic safety applications. Consequently, it is not necessary for theremote UE to monitor other discovery resource pools, thereby being ableto save battery.

Further Embodiments

According to a first aspect, a method is provided for activating a relayfunctionality of a relay user equipment within a mobile communicationnetwork. The relay user equipment is capable of performing directcommunication over a direct sidelink connection respectively with one ormore remote user equipments. The relay user equipment is located in aradio cell controlled by a radio base station in the mobilecommunication network and supports a relay functionality for beingcapable of serving as a relay, respectively for the one or more remoteuser equipments, so as to relay communication between the one or moreremote user equipments and the radio base station via the directsidelink connection. The method comprises the following steps. The radiobase station determines whether or not further relays are necessary inthe radio cell. In case it is determined that further relays arenecessary in the radio cell, the radio base station selects apersistence check value and transmits a broadcast message in the radiocell. This broadcast message at least indicates that further relays arenecessary in the radio cell and comprises the selected persistence checkvalue. The relay user equipment receives the broadcast message, and thenactivates its relay functionality in case the relay user equipmentdetermines that relay requirements for activating its relayfunctionality in the radio cell are fulfilled by the relay userequipment and in case a persistence check performed by the relay userequipment based on the received persistence check value is successful.

According to a second aspect which is provided in addition to the firstaspect, the step of performing by the relay user equipment thepersistence check comprises that the relay user equipment generates arandom value within a range of values and compares the generated randomvalue with the received persistence check value, having been selected bythe radio base station within the same range of values, to determinewhether the persistence check is successful or not. For example, in casethe generated random value is smaller than or equal to the receivedpersistence check value, the persistence check is successful.

According to a third aspect which is provided in addition to the firstor second aspect, the method may include further steps after receivingthe broadcast message and before the step of activating the relayfunctionality. In particular, the relay user equipment determineswhether radio resources are already configured for the relay userequipment to perform a relay discovery procedure to announce itspresence as a relay. In case no radio resources are already configuredfor the relay user equipment to perform the relay discovery procedure,the relay user equipment requests from the radio base station radioresources to perform the relay discovery procedure, and then receivesfrom the radio base station information on whether and which radioresources are assigned to perform the relay discovery procedure.

According to a fourth aspect which is provided in addition to any of thefirst to third aspects, the method may include further steps afterreceiving the broadcast message and before the step of activating therelay functionality. In particular, the relay user equipment transmitsto the radio base station a relay activation request message, requestingpermission from the radio base station to activate the relayfunctionality of the relay user equipment. The relay user equipmentreceives from the radio base station a relay activation responsemessage, giving or denying the permission for the relay user equipmentto activate the relay functionality. Then, the step of activating by therelay user equipment its relay functionality is performed in case ofreceiving the relay activation response message giving the permission toactivate the relay functionality.

According to a fifth aspect which is provided in addition to the thirdand fourth aspects, the step performed by the relay user equipment ofrequesting radio resources from the radio base station to perform therelay discovery procedure comprises including said request for radioresources in the relay activation request message transmitted by therelay user equipment to the radio base station to request the permissionfrom the radio base station to activate its relay functionality.Furthermore, the step performed by the relay user equipment of receivingfrom the radio base station the information on whether and which radioresources are assigned to perform the relay discovery procedurecomprises including said information in the relay activation responsemessage transmitted by the radio base station to give or deny thepermission for the relay user equipment to activate its relayfunctionality. For example, the radio base station, by assigning radioresources to the relay user equipment for the relay discovery procedure,gives the permission for the relay user equipment to activate its relayfunctionality, and wherein the radio base station, by not assigningradio resources to the relay user equipment for the relay discoveryprocedure, denies the permission for the relay user equipment toactivate its relay functionality.

According to a sixth aspect which is provided in addition to this fourthor fifth aspect, wherein the relay activation request message furthercomprises: 1) information on one or more services that can be providedby the relay user equipment to remote user equipments, such as publicsafety services or non-public safety services, preferably allowing theradio base station to determine a priority associated with the one ormore provided services, 2) group identification information of one ormore services that can be provided by the relay user equipment to remoteuser equipments, the group identification information giving informationas to which group each of the one or more provided services belongs to,or 3) a request for radio resources for the relay user equipment toperform direct discovery to announce its presence as a directcommunication user equipment by transmitting discovery messages in theradio cell.

According to a seventh aspect which is provided in addition to any ofthe first to sixth aspects, the method further comprises the followingsteps after receiving the broadcast message and before the step ofactivating the relay functionality. The relay user equipment determineswhether the relay user equipment is in an idle state or in a connectedstate. In case the relay user equipment is in an idle state, the relayuser equipment transitions to the connected state so as to be able torequest resources from the radio base station to perform a relaydiscovery procedure and/or to be able to request permission from theradio base station to activate the relay functionality of the relay userequipment. For example, the step of the relay user equipmenttransitioning to the connected state comprises performing by the relayuser equipment a connection request procedure with the radio basestation. This connection request procedure may indicate as anestablishment cause the need to request radio resources to perform therelay discovery procedure and/or the need to seek permission to activatethe relay functionality. Correspondingly, the radio base stationdetermines whether to deny or permit the connection request based on theestablishment cause. The establishment cause can be determined by theradio base station during the connection request procedure from a RadioResource Control protocol header of a message of the connection requestprocedure, or from a Medium Access Control protocol header of a messageof the connection request procedure or from a random access preambletransmitted by the relay user equipment during the connection requestprocedure.

According to an eighth aspect which is provided in addition to any ofthe first to seventh aspect, the broadcast message transmitted by theradio base station in the radio cell further comprises information onthe relay requirements to be fulfilled by relay user equipments in theradio cell. For example, the indication that further relays arenecessary is comprised in the broadcast message separately from theinformation on the relay requirements, or the relay user equipmentdetermines that further relays are necessary from the presence of theinformation on the relay requirements in the broadcast message and/orfrom the presence of the persistence check value in the broadcastmessage.

According to a ninth aspect which is provided in addition to any of thefirst to eighth aspects, the broadcast message transmitted by the radiobase station in the radio cell further comprises information on radioresources to be used by the relay user equipment for a relay discoveryprocedure to announce the presence of the relay user equipment as arelay. For example, the indication that further relays are necessary iscomprised in the broadcast message separately from the information onthe radio resources for the relay discovery procedure, or the relay userequipment determines that further relays are necessary from the presenceof the information on the radio resources for the relay discoveryprocedure in the broadcast message and/or from the presence of thepersistence check value in the broadcast message.

According to a tenth aspect which is provided in addition to any of thefirst to ninth aspect, the relay requirements comprise at least one ofthe following: 1) a minimum and/or maximum threshold for a radio linkquality of a link between the relay user equipment and the radio basestation, preferably wherein the radio link quality is determined basedon a reference signal receive power, RSRP, and/or a reference signalreceived quality, RSRQ, 2) a maximum threshold for a movement level ofthe relay user equipment, such as the speed of the relay user equipment,and 3) a minimum threshold for a battery level of the relay userequipment.

According to an eleventh aspect which is provided in addition to any ofthe first to tenth aspects, upon activating the relay functionality, therelay user equipment performs a relay discovery procedure to announceits presence as a relay in the radio cell by transmitting relaydiscovery messages in the radio cell. Each of the relay discoverymessages being transmitted either after receiving from a remote userequipment a relay solicitation message, which requests discovery ofrelay user equipments, or periodically. For example, the relay userequipment is selected to serve as a relay for a first remote userequipment to relay communication and a first direct sidelink connectionis established between the relay user equipment and the first remoteuser equipment such that communication exchanged by the first remoteuser equipment with the radio base station is relayed between the relayuser equipment and the first remote user equipment via the first directsidelink connection.

According to a twelfth aspect which is provided in addition to any ofthe first to eleventh aspect, the relay user equipment is assumed tohave its relay functionality activated, in which case the method furthercomprises the following steps. The relay user equipment receives fromthe radio base station a relay deactivation command and, in response,deactivates its relay functionality. Alternatively, the relay userequipment transmits to the radio base station a relay deactivationrequest message, and then receives, in response, from the radio basestation a relay deactivation response message that instructs the relayuser equipment to deactivate or not its relay functionality.Correspondingly, the relay user equipment deactivates its relayfunctionality in case the relay deactivation response message instructsto deactivate the relay functionality.

According to a thirteenth aspect which is provided in addition to any ofthe first to twelfth aspect, the method comprises the following steps.The relay user equipment receives from a proximity services function inthe mobile communication network a relay initiation message, and inresponse starts to monitor by the relay user equipment for receiving thebroadcast message from the radio base station. For example, the methodmay further comprise the following steps. The relay user equipmentreceives from the proximity services function a relay stop message, andin response stops to monitor for receiving the broadcast message fromthe radio base station.

According to a fourteenth aspect which is provided in addition to any ofthe first to thirteenth aspect, the step performed by the radio basestation to determine if further relays are necessary determines thatfurther relays are necessary in case of receiving a relay initiationmessage from a proximity services function in the mobile communicationnetwork. Alternatively or in addition, the step performed by the radiobase station to determine if further relays are necessary is based onthe number of remote user equipments in the radio cell that have a badradio link with the radio base station, and/or based on the number ofremote user equipments running public safety services in the radio cell.

According to a fifteenth aspect, a relay user equipment is providedwithin a mobile communication network for activating a relayfunctionality. The relay user equipment is capable of performing directcommunication over a direct sidelink connection respectively with one ormore remote user equipments. The relay user equipment is located in aradio cell controlled by a radio base station in the mobilecommunication network and supports a relay functionality for beingcapable of serving as a relay, respectively for the one or more remoteuser equipments, so as to relay communication between the one or moreremote user equipments and the radio base station via the directsidelink connection. A receiver of the relay user equipment receivesfrom the radio base station a broadcast message indicating that furtherrelays are necessary in the radio cell and comprising a persistencecheck value selected by the radio base station. A processor of the relayuser equipment activates, upon receiving the broadcast message, therelay functionality of the relay user equipment in case the relay userequipment determines that relay requirements for activating its relayfunctionality in the radio cell are fulfilled by the relay userequipment and in case a persistence check performed by the relay userequipment based on the received persistence check value is successful.

According to a sixteenth aspect which is provided in addition to thefifteenth aspect, the processor is configured to perform the persistencecheck by 1) generating a random value within a range of values, 2)comparing the generated random value with the received persistence checkvalue, having been selected by the radio base station within the samerange of values, to determine whether the persistence check issuccessful or not. For example, the processor determines that thepersistence check is successful in case the generated random value issmaller than or equal to the received persistence check value.

According to a seventeenth aspect which is provided in addition to thefifteenth or sixteenth aspect, the processor determines, after thereceiver receives the broadcast message and before the processoractivates the relay functionality, whether radio resources are alreadyconfigured for the relay user equipment to perform a relay discoveryprocedure to announce its presence as a relay. In case the processordetermines that no radio resources are already configured for the relayuser equipment to perform the relay discovery procedure, the processorrequests from the radio base station radio resources to perform therelay discovery procedure, and the receiver receives from the radio basestation information on whether and which radio resources are assigned toperform the relay discovery procedure.

According to an eighteenth aspect which is provided in addition to anyof the fifteenth to seventeenth aspect, after the receiver receives thebroadcast message and before the processor activates the relayfunctionality, a transmitter of the relay user equipment transmits tothe radio base station a relay activation request message, requestingpermission from the radio base station to activate the relayfunctionality of the relay user equipment. The receiver receives fromthe radio base station a relay activation response message, giving ordenying the permission for the relay user equipment to activate therelay functionality. The processor activates the relay functionality incase of receiving the relay activation response message giving thepermission to activate the relay functionality.

According to a nineteenth aspect provided in addition to the seventeenthand eighteenth aspects the processor requests the radio resources fromthe radio base station to perform the relay discovery procedure byincluding a request for radio resources in the relay activation requestmessage to be transmitted by the transmitter to the radio base stationto request the permission from the radio base station to activate itsrelay functionality. The receiver receives the relay activation responsemessage transmitted by the radio base station to give or deny thepermission for the relay user equipment to activate its relayfunctionality by receiving from the radio base station the informationon whether and which radio resources are assigned to perform the relaydiscovery procedure.

According to a twentieth aspect provided in addition to any of thefifteenth to nineteenth aspect, after the receiver receives thebroadcast message and the processor activates the relay functionality,the processor determines whether the relay user equipment is in an idlestate or in a connected state. In case the processor determines therelay user equipment to be in an idle state, the processor transitionsthe relay user equipment to the connected state so as to be able torequest resources from the radio base station to perform a relaydiscovery procedure and/or to be able to request permission from theradio base station to activate the relay functionality of the relay userequipment.

According to a 21st aspect provided in addition to any of the fifteenthto twentieth aspects, the receiver receives the broadcast messagecomprising information on the relay requirements to be fulfilled byrelay user equipments in the radio cell. In addition or alternatively,the receiver receives the broadcast message comprising information onradio resources to be used by the relay user equipment for a relaydiscovery procedure to announce the presence of the relay user equipmentas a relay.

According to a 22nd aspect provided in addition to any of the fifteenthto 21st aspect, the processor determines whether at least one of thefollowing relay requirements is fulfilled. A minimum and/or maximumthreshold for a radio link quality of a link between the relay userequipment and the radio base station, preferably wherein the radio linkquality is determined based on a reference signal receive power, RSRP,and/or a reference signal received quality, RSRQ; a maximum thresholdfor a movement level of the relay user equipment, such as the speed ofthe relay user equipment; and a minimum threshold for a battery level ofthe relay user equipment.

According to a 23rd aspect provided in addition to any of the fifteenthto 22nd aspect, the relay user equipment has activated its relayfunctionality, and the receiver receives from the radio base station arelay deactivation command. The processor deactivates, in response tothe relay deactivation command, the relay functionality. For example,the transmitter transmits to the radio base station a relay deactivationrequest message requesting the radio base station to deactivate or notthe reader functionality of the relay user equipment.

According to a 24th aspect provides a radio base station forparticipating in activating a relay functionality of a relay userequipment within a mobile communication network. The relay userequipment is capable of performing direct communication over a directsidelink connection respectively with one or more remote userequipments. The relay user equipment is located in a radio cellcontrolled by a radio base station in the mobile communication networkand supports a relay functionality for being capable of serving as arelay, respectively for the one or more remote user equipments, so as torelay communication between the one or more remote user equipments andthe radio base station via the direct sidelink connection. A processorof the radio base station determines whether or not further relays arenecessary in the radio cell. The processor selects a persistence checkvalue within a range of values. A transmitter transmits, to the one ormore remote user equipments in the radio cell, relay requirements to befulfilled before activating the relay functionality. The transmittertransmits a broadcast message in the radio cell in case the processordetermines that further relays are necessary. The broadcast message atleast indicates that further relays are necessary in the radio cell andcomprises the selected persistence check value. The broadcast messageindicates to the one or more relay user equipments in the radio cell toactivate the relay functionality, in case the relay user equipmentsuccessfully performs a persistence check based on the persistence checkvalue and in case that the relate user equipment fulfills the relayrequirements.

According to a 25th fifth aspect which is provided in addition to the24th aspect, the processor determines that further relays are necessaryin case the receiver receives a relay initiation message from aproximity services function in the mobile communication network.Additionally or alternatively, the processor determines that furtherrelays are necessary based on the number of remote user equipments inthe radio cell that have a bad radio link with the radio base stationand/or based on the number of remote user equipments running publicsafety services in the radio cell.

Hardware and Software Implementation of the Present Disclosure

Other exemplary embodiments relate to the implementation of the abovedescribed various embodiments using hardware, software, or software incooperation with hardware. In this connection a user terminal (mobileterminal) and an eNodeB (base station) are provided. The user terminaland base station is adapted to perform the methods described herein,including corresponding entities to participate appropriately in themethods, such as receiver, transmitter, processors.

It is further recognized that the various embodiments may be implementedor performed using computing devices (processors). A computing device orprocessor may for example be general purpose processors, digital signalprocessors (DSP), application specific integrated circuits (ASIC), fieldprogrammable gate arrays (FPGA) or other programmable logic devices,etc. The various embodiments may also be performed or embodied by acombination of these devices. In particular, each functional block usedin the description of each embodiment described above can be realized byan LSI as an integrated circuit. They may be individually formed aschips, or one chip may be formed so as to include a part or all of thefunctional blocks. They may include a data input and output coupledthereto. The LSI here may be referred to as an IC, a system LSI, a superLSI, or an ultra LSI depending on a difference in the degree ofintegration. However, the technique of implementing an integratedcircuit is not limited to the LSI and may be realized by using adedicated circuit or a general-purpose processor. In addition, a FPGA(Field Programmable Gate Array) that can be programmed after themanufacture of the LSI or a reconfigurable processor in which theconnections and the settings of circuits cells disposed inside the LSIcan be reconfigured may be used.

Further, the various embodiments may also be implemented by means ofsoftware modules, which are executed by a processor or directly inhardware. Also a combination of software modules and a hardwareimplementation may be possible. The software modules may be stored onany kind of computer readable storage media, for example RAM, EPROM,EEPROM, flash memory, registers, hard disks, CD-ROM, DVD, etc. It shouldbe further noted that the individual features of the differentembodiments may individually or in arbitrary combination be subjectmatter to another embodiment.

It would be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present disclosure asshown in the specific embodiments. The present embodiments are,therefore, to be considered in all respects to be illustrative and notrestrictive.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. An integrated circuit which, in operation, controls operation of auser equipment, the integrated circuit comprising: reception circuitry,which, in operation, receives a threshold value indicating a minimumvalue for a radio link quality of a first communication link with aradio base station; and control circuitry, which is coupled to thereception circuitry and which, in operation, performs a relay discoveryprocedure to discover a plurality of relay user equipments located in aradio cell, selects a relay user equipment out of the plurality ofdiscovered relay user equipments located in the radio cell, determineswhether to perform communication over the first communication linkbetween the user equipment and the radio base station or over a secondcommunication link between the user equipment and the selected relayuser equipment, based on the radio link quality of the firstcommunication link and the received threshold value, and performscommunication over the second communication link responsive to the radiolink quality of the first communication link being below the receivedthreshold value.
 2. The integrated circuit according to claim 1, whereinthe radio link quality of the first communication link is measured basedon a Reference Signal Receive Power (RSRP) and/or Reference SignalReceived Quality (RSRQ).
 3. The integrated circuit according to claim 1,wherein the control circuitry, in operation, performs communication overthe first communication link responsive to the radio link quality of thefirst communication link being higher than the received threshold value.4. The integrated circuit according to claim 3, wherein the controlcircuitry, in operation, performs a radio resource control (RRC)connection establishment procedure for the user equipment to switchcommunication from the second communication link to the firstcommunication link.
 5. The integrated circuit according to claim 1,wherein the control circuitry, in operation, performs communication overthe first communication link responsive to a cell selection criterionbeing fulfilled.
 6. The integrated circuit according to claim 1, whereinthe control circuitry, in operation, monitors specific discoveryresource pools of public safety applications for which the userequipment is configured.
 7. The integrated circuit according to claim 1,wherein the first communication link is Uu link and the secondcommunication link is PC5 link, as defined in the 3GPP technicalstandard.
 8. An integrated circuit configured to control a process of auser equipment, the process including: receiving a threshold valueindicating a minimum value for a radio link quality of a firstcommunication link with a radio base station; performing a relaydiscovery procedure to discover a plurality of relay user equipmentslocated in a radio cell, selecting a relay user equipment out of theplurality of discovered relay user equipments located in the radio cell,determining whether to perform communication over the firstcommunication link between the user equipment and the radio base stationor over a second communication link between the user equipment and theselected relay user equipment, based on the radio link quality of thefirst communication link and the received threshold value, andperforming communication over the second communication link responsiveto the radio link quality of the first communication link being belowthe received threshold value.
 9. The integrated circuit according toclaim 8, wherein the radio link quality of the first communication linkis measured based on a Reference Signal Receive Power (RSRP) and/orReference Signal Received Quality (RSRQ).
 10. The integrated circuitaccording to claim 8, wherein the process includes performingcommunication over the first communication link responsive to the radiolink quality of the first communication link being higher than thereceived threshold value.
 11. The integrated circuit according to claim10, wherein the process includes performing a radio resource control(RRC) connection establishment procedure for the user equipment toswitch communication from the second communication link to the firstcommunication link.
 12. The integrated circuit according to claim 8,wherein the process includes performing communication over the firstcommunication link responsive to a cell selection criterion beingfulfilled.
 13. The integrated circuit according to claim 8, wherein theprocess includes monitoring specific discovery resource pools of publicsafety applications for which the user equipment is configured.
 14. Theintegrated circuit according to claim 8, wherein the first communicationlink is Uu link and the second communication link is PC5 link, asdefined in the 3GPP technical standard.